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Metadata, conditional processing, and indexing
Condition (or select) attributes
The audience, platform, product, otherprops and rev attributes can be applied to most DITA elements. They are used for conditional processing, and are known as the condition (or select) attributes.
Controlled values for attributes
Controlled values for metadata attributes can be defined in a ditamap, and will apply when authoring topics referenced by the ditamap. These controlled values are defined in the subjectScheme element.

Introduction
Information types and topics
Topics are the building blocks of modular documents, and organising topics by semantic information types is one of the architectural features of DITA.
DITA map files
Syntax and mark-up
Language and punctuation
Graphics and figures
Cross-referencing
Cross-references are a means of binding individual topics into a coherent and navigable document.
Content re-use
Metadata, conditional processing, and indexing
- Conditional processing concepts
  When DITA content is processed to a reading format, rules to exclude or highlight some of the content can be defined in a ditaval file. This conditional processing requires metadata attributes to be set on DITA elements that may possibly be excluded or highlighted.
- Condition (or select) attributes
  The audience, platform, product, otherprops and rev attributes can be applied to most DITA elements. They are used for conditional processing, and are known as the condition (or select) attributes.
  - Attributes with multiple values
    You can have more than one value for a select attribute. The different values are stored in the one attribute, separated by spaces.
  - Controlled values for attributes
    Controlled values for metadata attributes can be defined in a ditamap, and will apply when authoring topics referenced by the ditamap. These controlled values are defined in the subjectScheme element.
  - Conventions for the otherprops attribute
    By using a convention for recording values in the otherprops attribute, you can extend the amount of metadata stored.
  - The props attribute
    The props attribute is only used for specialising new metadata attributes.
- Filtering and flagging
  Content marked with metadata attributes can be conditionally processed so that the content is filtered (excluded) or flagged (highlighted) in the published output.
- Other metadata
  In addition to the select attributes, DITA content includes many forms of metadata. These metadata elements and attributes are used to manage and organise topics, ditamaps and publications.
- Indexing
  DITA incorporates powerful features for indexing, including multi-level indexing, and see and see-also redirection. Index entries can be applied in the ditamap, in the topic prolog, and inline within the topic content.
The DITA documentation process
DITA authoring concepts
Moving from style-based authoring to a structured authoring methodology such as DITA requires a radically different approach to writing.

Controlled values for attributes

Controlled values for metadata attributes can be defined in a ditamap, and will apply when authoring topics referenced by the ditamap. These controlled values are defined in the subjectScheme element.

DITA 1.2 introduced the ability for the author to specify a set of valid metadata attribute values in the ditamap, and have the authoring tool validate the topics to ensure only listed attribute values were used.

The valid attribute values are stored in a subjectScheme section in the ditamap.

You should store the subjectScheme in a nested ditamap. This is because subjectScheme sections are likely to be re-used, and having separate files makes for easier organisation.

This sample mark-up shows controlled values for the platform attribute of linux, mswin, and zos:

<subjectScheme> 
  <subjectdef keys="os" navtitle="Operating system">
    <subjectdef keys="linux" navtitle="Linux"/>
    <subjectdef keys="mswin" navtitle="Microsoft Windows"/> 
    <subjectdef keys="zos" navtitle="z/OS"/>
  </subjectdef>
  <enumerationdef>
    <attributedef name="platform"/>
    <subjectdef keyref="os"/>
  </enumerationdef>
</subjectScheme>