application definition, extends NXobject


Characterization and session with one sample in an electron microscope.

The idea and aim of NXem: Electron microscopes (EM), whether it be a scanning electron microscope (SEM) or a transmission electron microscope (TEM), are versatile tools for preparing and characterizing samples and specimens. The term specimen is here understood as a synonym for a sample. A specimen is a physical portion of material that is studied/characterize in the microscope session, eventually in different places on the specimen surface. These places are named regions of interest (ROIs).

Fundamentally, an EM is an electron accelerator. Experimentalists use an EM in sessions during which they characterize as well as prepare specimens. This application definition describes data and metadata about processes and characterization tasks applied to one specimen.

Multiple specimens have to be described with multiple NXentry instances.

There are research groups who use an EM in a manner where it is exclusively operated by a single, instrument-responsible scientists or a team of (staff) scientists. These users perform analyses for other users as a service task. Oftentimes, though, and especially for cutting-edge instruments, the scientists and their team guide the process while operating the microscope. Oftentimes the scientists operate the instrument themselves either on-site or remotely and can ask technicians for support. In all cases, these people are considered users. Users might have different roles though.

The rational behind a common EM schema rather than separate SEM or TEM schemata are primarily the key similarities of SEM and TEM instruments: Both have electro-magnetic lenses. These lens may differ in design, alignment, number, and level of corrected-for aberrations. As an obvious difference, a TEM is used mainly to measure the transmitted electron beam. This demands thinner specimens as in SEM but offers capabilities for probing of additional physical mechanisms of electron-matter interaction.

Compared to SEMs, TEMs have a different relative arrangement between the lenses and the specimen which is most obvious by the different relative arrangement of the objective lens versus the specimen.

Nevertheless, both types of electron microscopes use detector systems which measure different types of signals that originate though from the same set of radiation/specimen interactions. Consequently, detectors can also be similar.

Given these physical and technical differences, different instruments have been developed. This led to a coexistence of two broad interacting communities: SEM and TEM users. From a data science perspective, we acknowledge that the more specific a research question is and the narrower the addressed user base is which develops or uses schemata for research data management with EM, the more understandable it is that scientists of either community (or sub-community) ask for method-specific schemata.

Researchers who have a single (main) microscope of some vendor in their lab, may argue they need an NXem_vendor_name schema or an NXem_microscope_name or an NXem_sem or a NXem_tem schema. Scientists exclusively working with one technique or type of signal probed (X-rays, electrons) may argue they wish to be pragmatic and store only what is immediately relevant for their particular technique and research questions. In effect, they may advocate for method-specific schemata such as NXem_ebsd, NXem_eels, NXem_edx, or NXem_imaging.

The development in the past has shown that these activities led to a zoo of schemata and implementations of these into many data and file formats. There is nothing which prevents the communities to make these schemata open and interoperable. Open here means specifically not that all data are compliant with/or use the schema and have to end up in the open-source domain. There can be embargo periods first of all. Open means that the metadata and associated schemata are documented in a manner that as many details as possible are open in the sense that others can understand what the (meta)data mean conceptually. The FAIR principles guide all decisions how data and metadata should be stored.

EM instruments, software, and research are moving targets. Consequently, there is a key challenge and inconvenience with having many different schemata with associated representations of data and metadata in EM: Each combination of schemata or an interoperable-made handshake between two file formats or software packages has to be maintained by software developers. This counts especially when data should be processed interoperably between software packages.

This brings two problems: Many software tools and parsers for the handshaking between tools to maintain. This can result in usage of different terminology. Which in turn results in representations and connections made between different data representations and workflows that are not machine-actionable. There are community efforts to harmonize the terminology.

A common vocabulary can serve interoperability as developers of schemata and scientists can take for instance then these terms as closely as possible. Ideally, they specialize the application definition only for the few very specific additional quantities of their instruments and techniques. This is better than reimplementing the wheel for descriptions of EM instruments. This route of more standardization can support the EM community in that it removes the necessity for having to maintain a very large number of schemata.

Aiming for more standardization, i.e. a lower number of schemata rather than a single standard for electron microscopy is a compromise that can serve academia as it enables the EM community to focus their software development efforts on those schemata, on fixing and discussing them, and on harmonize their common vocabulary. These activities can be specifically relevant also for vendors of EM hard- and software as it improves the longevity of certain schema and thus can help to incentivize vendors to support the community with implementing support for such schemata into their proprietary applications.

In effect, everybody can gain from this as it will likely reduce the cases in which scientists have to fix bugs in making their own tools compliant and interoperable with tools of their colleagues and the wider community.

The here proposed NXem application definition offers modular components (EM-research specific base classes) for using NeXus to define schemata for electron microscopy research. Working towards a common vocabulary is a community activity that profits from everybody reflecting in detail whether certain terms they have used are not eventually conceptually similar if not the same as what this application definition and its base classes provide.

We are happy for receiving your feedback.

It is noteworthy to understand that (not only for) NeXus, schema differ already if at least one field is required in one version of the schema, but it is set optional in another version. If group(s), field(s), or attributes are removed or added, or even a docstring is changed, schemata can become inconsistent. An application definition here serves as a contract between a data provider and a data consumer. These two can be software tools (like the vendor software to drive the instrument or a scientific software for doing artificial intelligence with EM data). Such changes of a schema lead to new versions.

Tools like NeXus do not avoid or protect against inconsistencies; however NeXus offers a mechanism and toolset, through which schemata can be documented and defined. In effect, having an openly documented (at a case-specific level of technical detail) schema is a necessary but alone not a sufficient step to take EM research on a route of machine-actionable and interoperable FAIR data. A common vocabulary and a machine-actionable knowledge representation/engine is also required. Essentially when the docstrings are no longer needed but can be replaced by a connection to an automated tool which understands what a specific field represents conceptually, EM data have become more generally interoperable EM data.

This application definition takes a key step into this direction. It offers a controlled vocabulary and relation between concepts and data relevant for research with electron microscopes. To be most efficient and offering reusability, the application definition should be understood as a template that one should ideally use as is. This application definition is called NXem. It can be considered a base for more specialized definitions (ideally prefixed with NXem) method.

The use of NXem should be as follows: Offspring application definitions should not remove groups but make them optional or, even better, propose changes in the application definition.

A particular challenge with electron microscopes as physical instruments are their dynamics. To make EM data understandable, repeatable, and eventually corresponding experiments reproducible in general requires a documentation of the spatio-temporal dynamics of the instrument in its environment. For most commercial systems there is a specific accessibility beyond which detailed settings like lens excitations and low-level hardware settings may not be retrievable.

EM experiments by design illuminate the specimen with electrons as a consequence of which the specimen changes if not may get destroyed. As such, repeatability of numerical processing and clear descriptions of procedures and system setups should be addressed first.

If especially a certain simulation package needs a detailed view of the geometry of the lens system and its excitations during the course of the experiment, it is difficult to fully abstract the technical details of the hardware into a set of names for fields and groups that make for a compromise between clarity and being vendor-agnostic. Settings of apertures are an example where aperture modes are aliases behind which there is a set of settings. These settings are difficult to retrieve, often undocumented in detail. This serves users and makes EM experiments easier understandable and conveniently executable for a broader user base. The opportunities for application definitions to offer an abstraction layer are limited.

Instead, currently it is for the docstring to specify what is conceptually eventually behind such aliases. The design rule we followed while drafting the application definition and base classes is that there are numerous (technical) details about an EM which may warrant a very detailed technical disentangling of settings and reflection of numerous settings as deeply nested groups, fields and attributes. An application definition can offer a place to hold these nested representations; however at the cost of generality.

Which specific details matter for answering scientific research questions is a difficult question to answer by a single team of scientists, especially if the application definition is to speak for a number of vendors. What makes it especially challenging if the application definition is expected to hold all data that might be of relevance for future questions.

We are skeptical if there is one representation that can fulfill all these aims, while remaining at the same time approachable and executable by a large number of scientists in a community. With this application definition we would like to motivate the community to work towards such aim. While doing so we found that existent terminology can be encoded into a more controlled vocabulary.

We have concluded that despite all these details of current EM research with SEM, TEM, and focused-ion beam instruments, there a clearly identifiable common components and generalizable settings of EM research use cases.

This application definition has the following components at the top-level:

  • Generic experimental details (timestamp, identifiers, name); conceptually these are session details. A session at a microscope may involve the characterization of multiple specimens. For each specimen an instance of an (NXentry) is created. Details of the instrument have to be stored at least in an entry. Other entries should refer to these metadata via links to reduce redundancies.

  • Each signal, such as a spectrum or image taken at the microscope, should have an associated time stamp and report of the specific settings at that point in time when the image was taken. The reason is that EMs can be highly dynamic, be used to illuminate the specimen differently or show drift during signal acquisition, to name but a few effects. What constitutes a single EM experiment/measurement? This can be the collecting of a single diffraction pattern with a scanning TEM (STEM), taking of a secondary electron image for fracture analysis, taking a set of EBSD line scan and surface mappings in an SEM, or ion-beam-milling of a specimen in preparation for an atom probe experiment.

  • NXmonitor; instances to keep track of time-dependent quantities pertaining to specific components of the instrument. Alternatively NXevent_data_em instances can be used to store timestamp states of the components, which is relevant to document the exact settings when images and spectra were taken.

  • NXinstrument; conceptually this is a container to store arbitrary level of detail of the technical components of the microscope as a device and the lab in which it is operated.

  • NXuser; conceptually, this is a set with at least one NXuser instance which details who operated or performed the measurement. Additional NXusers can be referred to in an NXevent_data_em instance to store individualized details who executed an event.

  • NXevent_data_em instances as an NXevent_data_em_set; each NXevent_data_em instance is a container to group specific details about the state of the microscope when a measurement was taken and relevant data and eventual processing steps were taken (on-the-fly).

  • NXdata; a the top-level, conceptually, this is a place for documenting available default plottable data. A default plottable can be useful for research data management systems to show a visual representation of some aspect of the content of the EM session. It is clear that what constitutes a useful default plot is a matter of interpretation, somewhat of personal taste, and community standards.

In effect, default plottables are case- and method-specific. Usually a session at a microscope is used to collect multiple signals and images. Examples for possible default plottables could be an arbitrarily taken: secondary, back-scattered, electron image, diffraction pattern, EELS spectra, composition, or orientation mappings to name but a few.

There are a few design choices to consider with sub-ordinate groups:

  • Above images, spectra, and mappings should be stored as NXdata instances, ideally formatted in such a way that they can be displayed with visualization software that can be specific for the file format in which the data are stored. NeXus specifies only the data model, i.e. the terms and their relations. These descriptions can be implemented and stored in JSON, HDF5, XML, or HSDS, file storage, or even other formats, although HDF5 is the most commonly used.

  • Consumable results of EM characterization tasks are usually a sub-set of data artifacts, as there is not an infinite amount of possible electron/ion beam-specimen interactions.

  • Images of electron counts detected in specific operation modes (bright field, dark field in TEM, secondary/back-scattered, Kikuchi in SEM)

  • Spectra (X-ray quanta or auger electron counts)

  • These data are in virtually all cases a result of some numerical processing. It makes sense to name them with a controlled vocabulary, e.g. SE (secondary electron), BSE (back-scattered electron), Kikuchi, X-ray, Auger, Cathodolum(inescence) etc.

A key question often asked with EM experiments is how the actual (meta)data should be stored (in memory or on disk). To this end the schema, here makes no specific assumptions, not even that all the fields/group of a schema instance have to be stored into a single file. Instead, the schema specifies the relations between metadata, constraints on how they should be formatted, what they conceptually represent and which terms (controlled vocabulary) is practical to store with the data.

In effect, the application definition is a graph which describes how (meta)data are related to one another.


No symbol table

Groups cited:

NXcoordinate_system_set, NXdata, NXdetector, NXebeam_column, NXentry, NXevent_data_em_set, NXevent_data_em, NXibeam_column, NXimage_set_em_adf, NXimage_set_em_bf, NXimage_set_em_bse, NXimage_set_em_chamber, NXimage_set_em_df, NXimage_set_em_diffrac, NXimage_set_em_ecci, NXimage_set_em_kikuchi, NXimage_set_em_ronchigram, NXimage_set_em_se, NXinstrument, NXmanufacturer, NXmonitor, NXnote, NXoptical_system_em, NXpump, NXsample, NXscanbox_em, NXspectrum_set_em_auger, NXspectrum_set_em_cathodolum, NXspectrum_set_em_eels, NXspectrum_set_em_xray, NXuser


ENTRY: (required) NXentry

@version: (required) NX_CHAR

An at least as strong as SHA256 hashvalue of the file that specifies the application definition.

definition: (required) NX_CHAR

NeXus NXDL schema to which this file conforms.

Obligatory value: NXem

experiment_identifier: (required) NX_CHAR

Ideally, a (globally) unique persistent identifier for referring to this experiment.

The identifier is usually defined/issued by the facility, laboratory, or the principle investigator. The identifier enables to link experiments to e.g. proposals.

experiment_description: (optional) NX_CHAR

Free-text description about the experiment.

Users are strongly advised to detail the sample history in the respective field and fill rather as completely as possible the fields of this application definition rather than write details about the experiment into this free-text description field.

start_time: (required) NX_DATE_TIME

ISO 8601 time code with local time zone offset to UTC information included when the microscope session started. If the application demands that time codes in this section of the application definition should only be used for specifying when the experiment was performed - and the exact duration is not relevant - this start time field should be used.

Often though it is useful to specify a time interval with specifying both start_time and end_time to allow for more detailed bookkeeping and interpretation of the experiment. The user should be aware that even with having both time instances specified, it may not be possible to infer how long the experiment took or for how long data were acquired.

More detailed timing data over the course of the experiment have to be collected to compute this. These computations can take advantage of individual time stamps in NXevent_em instances to provide additional pieces of information.

end_time: (required) NX_DATE_TIME

ISO 8601 time code with local time zone offset to UTC included when the microscope session ended.

program: (required) NX_CHAR

Commercial or otherwise given name to the program which was used to create the file.

Electron microscopy experiments are usually controlled/performed via commercial integrated acquisition and instrument control software. In many cases, an EM dataset is useful only if it gets post-processed already during the acquisition, i.e. while the scientist is sitting at the microscope. Many of these processes are automated, while some demand GUI interactions with the control software. Examples include collecting of diffraction pattern and on-the-fly indexing of these.

It is possible that different types of programs might be used to perform these processing steps whether on-the-fly or not. If this is the case the processing should be structured with individual NXprocess instances. If the program and/or version used for processing referred to in an NXprocess group is different to the program and version mentioned in this field, the NXprocess needs to hold an own program and version.

@version: (required) NX_CHAR

Program version plus build number, commit hash, or description of an ever persistent resource where the source code of the program and build instructions can be found so that the program can be configured in such a manner that the result file is ideally recreatable yielding the same results.

experiment_documentation: (optional) NXnote

Binary container for a file or a compressed collection of files which can be used to add further descriptions and details to the experiment. The container can hold a compressed archive.

thumbnail: (optional) NXnote

A small image that is representative of the entry; this can be an image taken from the dataset like a thumbnail of a spectrum. A 640 x 480 pixel jpeg image is recommended. Adding a scale bar to that image is recommended but not required as the main purpose of the thumbnail is to provide e.g. thumbnail images for displaying them in data repositories.

@type: (required) NX_CHAR

operator: (required) NXuser

Contact information and eventually details of at least one person involved in the taking of the microscope session. This can be the principle investigator who performed this experiment. Adding multiple users if relevant is recommended.

name: (required) NX_CHAR

Given (first) name and surname of the user.

affiliation: (recommended) NX_CHAR

Name of the affiliation of the user at the point in time when the experiment was performed.

address: (recommended) NX_CHAR

Postal address of the affiliation.

email: (required) NX_CHAR

Email address of the user at the point in time when the experiment was performed. Writing the most permanently used email is recommended.

orcid: (recommended) NX_CHAR

Globally unique identifier of the user as offered by services like ORCID or ResearcherID.

telephone_number: (optional) NX_CHAR

(Business) (tele)phone number of the user at the point in time when the experiment was performed.

role: (optional) NX_CHAR

Which role does the user have in the place and at the point in time when the experiment was performed? Technician operating the microscope. Student, postdoc, principle investigator, guest are common examples.

social_media_name: (optional) NX_CHAR

Account name that is associated with the user in social media platforms.

social_media_platform: (optional) NX_CHAR

Name of the social media platform where the account under social_media_name is registered.

SAMPLE: (required) NXsample

A description of the material characterized in the experiment. Sample and specimen are threaded as de facto synonyms.

method: (required) NX_CHAR

A qualifier whether the sample is a real one or a virtual one (in a computer simulation)

Any of these values: experimental | simulation

name: (required) NX_CHAR

Descriptive name or ideally (globally) unique persistent identifier. The name distinguishes the specimen from all others and especially the predecessor/origin from where the specimen was cut.

This field must not be used for an alias of the sample. Instead, use short_title.

In cases where multiple specimens have been loaded into the microscope the name has to identify the specific one, whose results are stored by this NXentry, because a single NXentry should be used only for the characterization of a single specimen.

Details about the specimen preparation should be stored in the sample history.

sample_history: (required) NX_CHAR

Ideally, a reference to a (globally) unique persistent identifier, representing a data artifact which documents ideally as many details of the material, its microstructure, and its thermo-chemo-mechanical processing/preparation history as possible.

The sample_history is the record what happened before the specimen was placed into the microscope at the beginning of the session.

In the case that such a detailed history of the sample/specimen is not available, use this field as a free-text description to specify a sub-set of the entire sample history, i.e. what you would consider are the key steps and relevant information about the specimen, its material, microstructure, thermo-chemo-mechanical processing state, and the details of the preparation.

Specific details about eventual physically-connected material like embedding resin should be documented ideally also in the sample_history. If all fails, the description field can be used but it is strongly discouraged because it leads to eventually non-machine-actionable data.

preparation_date: (required) NX_DATE_TIME

ISO 8601 time code with local time zone offset to UTC information when the specimen was prepared.

Ideally report the end of the preparation, i.e. the last known time the measured specimen surface was actively prepared. Usually this should be a part of the sample history, i.e. the sample is imagined handed over for the analysis. At the point it enters the microscope the session starts.

Knowing when the specimen was exposed to e.g. specific atmosphere is especially required for environmentally sensitive material such as hydrogen charged specimens or experiments including tracers with a short half time. Further time stamps prior to preparation_date should better be placed in resources which describe the sample_history.

short_title: (optional) NX_CHAR

Possibility to give an abbreviation or alias of the specimen name field.

atom_types: (required) NX_CHAR

Use Hill’s system for listing elements of the periodic table which are inside or attached to the surface of the specimen and thus relevant from a scientific point of view.

The purpose of the field is to offer materials database systems an opportunity to parse the relevant elements without having to interpret these from the sample history.

thickness: (required) NX_FLOAT {units=NX_LENGTH}

(Measured) sample thickness. The information is recorded to qualify if the beam used was likely able to shine through the specimen.

description: (optional) NX_CHAR

Discouraged free-text field in case properly designed records for the sample_history are not available.

DATA: (required) NXdata

Hard link to a location in the hierarchy of the NeXus file where the data for default plotting are stored.

COORDINATE_SYSTEM_SET: (required) NXcoordinate_system_set

MONITOR: (optional) NXmonitor

em_lab: (required) NXinstrument

Metadata and numerical data of the microscope and the lab in which it stands.

The em_lab section contains a description of the instrument and its components. The component descriptions in this section differ from those inside individual NXevent_em sections. These event instances take the role of time snapshot. For an NXevent_em instance users should store only those settings for a component which are relevant to understand the current state of the component. Here, current means at the point in time, i.e. the time interval, which the event represents.

For example it is not relevant to store in each event’s electron_gun group again the details of the gun type and manufacturer but only the high-voltage if for that event the high-voltage was different. If for all events the high-voltage was the same it is not even necessary to include an electron_gun section in the event.

Individual sections of specific type should have the following names:

  • NXaperture: the name should match with the name of the lens

  • NXlens_em: condenser_lens, objective_lens are commonly used names

  • NXcorrector_cs: device for correcting spherical aberrations

  • NXstage_lab: a collection of component for holding the specimen and eventual additional component for applying external stimuli on the sample

  • NXdetector: several possible names like secondary_electron, backscattered_electron, direct_electron, ebsd, edx, wds, auger, cathodoluminescence, camera, ronchigram

instrument_name: (required) NX_CHAR

Given name of the microscope at the hosting institution. This is an alias. Examples could be NionHermes, Titan, JEOL, Gemini, etc.

location: (optional) NX_CHAR

Location of the lab or place where the instrument is installed. Using GEOREF is preferred.

MANUFACTURER: (required) NXmanufacturer

EBEAM_COLUMN: (required) NXebeam_column

IBEAM_COLUMN: (optional) NXibeam_column

ebeam_deflector: (required) NXscanbox_em

ibeam_deflector: (optional) NXscanbox_em

OPTICAL_SYSTEM_EM: (optional) NXoptical_system_em

DETECTOR: (required) NXdetector

Description of the type of the detector.

Electron microscopes have typically multiple detectors. Different technologies are in use like CCD, scintillator, direct electron, CMOS, or image plate to name but a few.

description: (optional) NX_CHAR

Free text option to write further details about the detector.

MANUFACTURER: (required) NXmanufacturer

PUMP: (optional) NXpump

measurement: (optional) NXevent_data_em_set

A container to structure a set of NXevent_em instances.

An event is a time point/interval during which the microscope was configured in a specific way and the microscope was used to take a measurement.

Each NXevent_em holds an acquisition task with the microscope. For instance the capturing of a secondary electron, backscattered electron, diffraction image, or spectrum.

An NXevent_em_data instance holds specific details about how raw data from a detector were processed into consumable data like images, spectra, etc. These on-the-fly data processing tasks are usually performed by the control software, eventually realized with custom scripts.

Furthermore, NXevent_em_state instances can document specific values and settings of the microscope during the snapshot/event.

EVENT_DATA_EM: (required) NXevent_data_em

A container holding a specific result of the measurement and eventually metadata how that result was obtained numerically.

NXevent_em instances can hold several specific NXimage_em or NXspectrum_em instances taken and considered as one event, i.e. a point in time when the microscope had the settings specified either in NXinstrument or in this NXevent_data_em instance.

The application definition is designed without an explicit need an NXevent_data_em instance that contains an NXimage_em or NXspectra_em instance. An NXevent_data_em can be used to document a specific state of the microscope at a time without having it placed into the NXinstrument group.

In other words the NXinstrument group details primarily the more static settings and components of the microscope as they are found by the operator during the session. The NXevent_data_em samples the dynamics.

It is not necessary to store data in NXebeam, NXibeam instances of NXevent_data_em but in this case it is assumed that the settings were constant over the entire course of microscope session and thus all relevant metadata inside the NXinstrument groups are sufficient to understand the session.

start_time: (required) NX_DATE_TIME

end_time: (required) NX_DATE_TIME

event_identifier: (required) NX_CHAR

Reference to a specific state and setting of the microscope components.

event_type: (required) NX_CHAR

detector_identifier: (required) NX_CHAR

The detector or set of detectors that was used to collect this signal. The name of the detector has to match one of the names of available NXdetector instances e.g. if the instrument has an ebsd_camera the detector for an NXimage_em_kikuchi should be the NXdetector instance called ebsd_camera.

IMAGE_SET_EM_SE: (optional) NXimage_set_em_se

IMAGE_SET_EM_BSE: (optional) NXimage_set_em_bse

IMAGE_SET_EM_ECCI: (optional) NXimage_set_em_ecci

IMAGE_SET_EM_BF: (optional) NXimage_set_em_bf

IMAGE_SET_EM_DF: (optional) NXimage_set_em_df

IMAGE_SET_EM_ADF: (optional) NXimage_set_em_adf

IMAGE_SET_EM_KIKUCHI: (optional) NXimage_set_em_kikuchi

IMAGE_SET_EM_DIFFRAC: (optional) NXimage_set_em_diffrac

SPECTRUM_SET_EM_XRAY: (optional) NXspectrum_set_em_xray

SPECTRUM_SET_EM_EELS: (optional) NXspectrum_set_em_eels

SPECTRUM_SET_EM_AUGER: (optional) NXspectrum_set_em_auger

SPECTRUM_SET_EM_CATHODOLUM: (optional) NXspectrum_set_em_cathodolum

IMAGE_SET_EM_RONCHIGRAM: (optional) NXimage_set_em_ronchigram

IMAGE_SET_EM_CHAMBER: (optional) NXimage_set_em_chamber

EBEAM_COLUMN: (optional) NXebeam_column

IBEAM_COLUMN: (optional) NXibeam_column

ebeam_deflector: (optional) NXscanbox_em

ibeam_deflector: (optional) NXscanbox_em

OPTICAL_SYSTEM_EM: (optional) NXoptical_system_em

USER: (optional) NXuser

Hypertext Anchors

List of hypertext anchors for all groups, fields, attributes, and links defined in this class.

NXDL Source: