# 3.1. Introduction to NeXus definitions¶

While the design principles of NeXus are explained in the NeXus: User Manual, this Reference Documentation specifies all allowed base classes and all standardized application definitions. Furthermore, it also contains contributed definitions of new bases classes or application definitions that are currently under review.

Base class definitions and application definitions have basically the same structure, but different semantics: Base class definitions define the complete set of terms that might be used in an instance of that class. Application definitions define the minimum set of terms that must be used in an instance of that class.

Base classes and application definitions are specified using a domain-specific XML scheme, the NXDL: The NeXus Definition Language.

## 3.1.1. Overview of NeXus definitions¶

For each class definition, the documentation is derived from content provided in the NXDL specification.

The documentation for each class consists of:

1. short table:

• the current version of the NXDL specification used for the class
• the category of the class (base class / application definition / contributed definition)
• The NeXus class extended by this class. Most NeXus base classes only extend the base class definition (NXDL).
• any other base classes (groups) cited by this class
2. symbol list:

keywords used to designate array dimensions. At present, this list is not guaranteed to be complete (some array dimension names appear only in the description column of the class member table, and not here)

3. source:

a link to the authorative NXDL source

4. tree outline:

hierarchical list of members.

5. member table:

list of top-level members with natural-language annotations.

6. supplementary member tables as needed:

member tables of subgroups.

## 3.1.2. Tree outlines¶

A compact listing of the basic structure (groups, fields, dimensions, attributes, and links) is prepared for each NXDL specification. Indentation shows nested structure. Attributes are prepended with the @ symbol. Links use the characters --> to represent the path to the intended source of the information.

## 3.1.3. Member tables¶

Member tables provide basic information about each field or group in the class. An example of the varieties of specifications are given in the following table using items found in various NeXus base classes.

Name Type Units Description (and Occurrences)
program_name NX_CHAR   Name of program used to generate this file
@version NX_CHAR

Program version number

Occurences: 1 : default

@configuration NX_CHAR   configuration of the program
thumbnail NXnote   A small image that is representative of the entry. An example of this is a 640x480 JPEG image automatically produced by a low resolution plot of the NXdata.
@mime_type NX_CHAR   expected: mime_type=”image/*”
NXgeometry   describe the geometry of this class
distance NX_FLOAT NX_LENGTH Distance from sample
mode “Single Bunch” | “Multi Bunch”   source operating mode
target_material Ta | W | depleted_U | enriched_U | Hg | Pb | C   Pulsed source target material

The columns in the table are described as follows:

Name (and attributes):

Name of the field. Since name needs to be restricted to valid program variable names, no “-” characters can be allowed. Name must satisfy both HDF and XML naming.

 1 2 3 4 5 6  NameStartChar ::= _ | a..z | A..Z NameChar ::= NameStartChar | 0..9 Name ::= NameStartChar (NameChar)* Or, as a regular expression: [_a-zA-Z][_a-zA-Z0-9]* equivalent regular expression: [_a-zA-Z][\w_]* 

Attributes, identified with a leading “at” symbol (@) and belong with the preceding field or group, are additional metadata used to define this field or group. In the example above, the program_name element has two attributes: version (required) and configuration (optional) while the thumbnail element has one attribute: mime_type (optional).

For groups, the name may not be declared in the NXDL specification. In such instances, the value shown in parentheses in the Name and Attributes column is a suggestion, obtained from the group by removing the “NX” prefix. See NXentry for examples.

Type:

Type of data to be represented by this variable. The type is one of those specified in NXDL: The NeXus Definition Language. In the case where the variable can take only one value from a known list, the list of known values is presented, such as in the target_material field above: Ta | W | depleted_U | enriched_U | Hg | Pb | C. Selections with included whitespace are surrounded by quotes. See the example above for usage.

For fields, the data type may not be specified in the NXDL file. The default data type is NX_CHAR and this is shown in parentheses in the Type column. See NXdata for examples.

Units:

Data units, given as character strings, must conform to the NeXus units standard. See the NeXus units section for details.

Description (and Occurrences):

A simple text description of the field. No markup or formatting is allowed. The absence of Occurrences in the item description signifies that both minOccurs and maxOccurs have the default values. If the number of occurrences of an item are specified in the NXDL (through @minOccurs and @maxOccurs attributes), they will be reported in the Description column similar to the example shown above. Default values for occurrences are shown in the following table. The NXDL element type is either a group (such as a NeXus base class), a field (that specifies the name and type of a variable), or an attribute of a field or group. The number of times an item can appear ranges between minOccurs and maxOccurs. A default minOccurs of zero means the item is optional. For attributes, maxOccurs cannot be greater than 1.

NXDL element type minOccurs maxOccurs
group [1] unbounded
field [1] unbounded
attribute [1] 1
 [1] (1, 2, 3) For NXDL base classes, minOccurs=0 is the default, for NXDL application definitions and contributed definitions, minOccurs=1 is the default. In all cases, the minOccurs attribute in the NXDL file will override the default for that element (group, field, attribute, or link).