Defining controlled values for attributes

Subject scheme maps can define controlled values for DITA attributes without having to define specializations or constraints. The list of available values can be modified quickly to adapt to new situations.

Each controlled value is defined using a <subjectdef> element, which is a specialization of the <topicref> element. The <subjectdef> element is used to define both a subject category and a list of controlled values. The parent <subjectdef> element defines the category, and the children <subjectdef> elements define the controlled values.

The subject definitions can include additional information within a <topicmeta> element to clarify the meaning of a value:

The <navtitle> element can provide a more readable value name.
The <shortdesc> element can provide a definition.

In addition, the <subjectdef> element can reference a more detailed definition of the subject, for example, another DITA topic or an external resource..

The following behavior is expected of processors:

Authoring tools SHOULD use these lists of controlled values to provide lists from which authors can select values when they specify attribute values.
Authoring tools MAY give an organization a list of readable labels, a hierarchy of values to simplify selection, and a shared definition of the value.
An editor MAY support accessing and displaying the content of the subject definition resource in order to provide users with a detailed explanation of the subject.
Tools MAY produce a help file, PDF, or other readable catalog to help authors better understand the controlled values.

Example: Controlled values that provide additional information about the subject

The following code fragment illustrates how a subject definition can provide a richer level of information about a controlled value:

<subjectdef keys="terminology" href="https://www.oasis-open.org/policies-guidelines/keyword-guidelines">
  <subjectdef keys="rfc2119" href="rfc-2119.dita">
    <topicmeta>
      <navtitle>RFC-2119 terminology</navtitle>
      <shortdesc>The normative terminology that the DITA TC uses for the DITA specification</shortdesc>
    </topicmeta>
  </subjectdef>
  <subjectdef keys="iso" href="iso-terminology.dita">
    <topicmeta>
      <navtitle>ISO keywords</navtitle>
      <shortdesc>The normative terminology used by some other OASIS technical committees</shortdesc>
    </topicmeta>
  </subjectdef>
</subjectdef>

The content of the <navtitle> and <shortdesc> elements provide additional information that a processor might display to users as they select attribute values or classify content. The resources referenced by the @href attributes provide even more detailed information; a processor might render clickable links as part of a user interface that implements a progressive disclosure strategy