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Home > Appendix > DITA 1.3 specification > Language reference: All-inclusive edition > Classification elements > Subject scheme elements
Subject scheme maps are used to define sets of controlled values and taxonomic subjects; bind controlled values to attributes; and specify relationships among taxonomic subjects.
<subjectScheme> element is a specialization of <map>; it defines taxonomic subjects and controlled values.<schemeref> element references another subject scheme map. Typically, the referenced subject scheme defines a base set of controlled values; the referencing map then extends the base set of controlled values. The values specified in the subject scheme maps are merged; the result is equivalent to specifying all of the values in a single map.<hasInstance> element specifies that the contained subjects have an INSTANCE-OF relationship with the container subject. In an INSTANCE-OF hierarchy, the child subject is a specific entity or object and the parent subject is a type, kind, or class of entity or object.<hasKind> element specifies that the contained hierarchy expresses KIND-OF relationships between subjects.<hasNarrower> element, the container subject is more general than each of the contained subjects. That is, this element makes the default hierarchical relationship explicit, although the way in which a relationship is narrower is not specified.<hasPart> element specifies that the contained hierarchy expresses PART-OF relationships between subjects.<hasRelated> element identifies an associative relationship between the container subject and each of the contained subjects.<enumerationdef> element identifies one attribute and one or more categories that contain the controlled values for the enumeration. The @type attribute has a default value of "keys". <elementdef> element identifies an element to which an attribute is bound. If the <enumerationdef> element does not contain an <elementdef> element, the enumeration is bound to the attribute in all elements.<attributedef> element specifies the attribute that is bound to a set of controlled values. This binding restricts the permissible values for the attribute to those that are contained in the set of controlled values.<defaultSubject> element is used within an attribute enumeration to set the default value for the attribute in cases where no value is specified for the attribute. The default subject must be one of the controlled values that are bound to the attribute.<subjectHead> element provides a heading for a group of subjects, for use if the scheme is displayed. For instance, a scheme might be displayed to let a user select subjects as part of faceted browsing. The <subjectHead> element itself does not reference a file and cannot be referenced as a key, so it does not define any controlled values.<subjectHeadMeta> element allows a navigation title and short description to be associated with a subject heading. <subjectdef> element defines a subject (also known as a controlled value) within a subject scheme. The @keys attribute specified on the <subjectdef> element assigns a key to the subject. A subject with a key can be addressed using a @keyref attribute.<relatedSubjects> element establishes associative relationships between each child subject and every other child subject (unless the association is restricted by the @linking attribute of the subjects).<subjectRelTable> element is a specialized relationship table which establishes relationships between the subjects in different columns of the same row. This element provides an efficient way to author non-hierarchical relationships between subjects. Tools (such as search tools) that use subject relationships to find related content might use these associative relationships in a similar way to the hierarchical relationships.<subjectRelHeader> element specifies the roles played by subjects in associations.<subjectRel> element contains a set of subjects that are related in some manner. Each group of subjects is contained in a <subjectRole> element; the associations between different columns in the same row are evaluated in the same way as those in a <relrow> (from which <subjectRel> is specialized) but define relationships between the subjects instead of links between topic documents.<subjectRole> element, when used within a <subjectRel> element, contains a set of subjects that are related to other subjects in the same row of the current <subjectRelTable>. By default, no relationship is defined between multiple subjects in the same <subjectRole> element. When used within the <subjectRelHeader>, the <subjectRole> element defines the category of subject or relationship provided by that column.