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Metadata, conditional processing, and indexing
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.
Nesting index entries
The technique of nesting indexterm elements can produce multi-level indexes in output documents.

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.
- 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.
  - Placement of indexterm elements
    The indexterm element is pure metadata, and does not display. You should place index entries in the prolog section of the topic, or in the topicmeta section of the topicref in the ditamap only if the term is specific to a particular publication.
  - Nesting index entries
    The technique of nesting indexterm elements can produce multi-level indexes in output documents.
  - Index see and see also
    Readers can be re-directed from a searched-for term to related terms in the index that provide further references. You can use the index-see and index-see-also indexing elements for this purpose.
  - Re-using index entries
    You can create a special ditamap which has a sole purpose of being the container for common indexterm elements for re-use in the prolog section of topics (and the topicmeta section of ditamaps, if required) through the conref feature.
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.

Nesting index entries

The technique of nesting indexterm elements can produce multi-level indexes in output documents.

You can created multi-level (secondary and tertiary) indexes by nesting indexterm elements.

For example, the mark-up:

<indexterm>EJ25 engine
  <indexterm>performance</indexterm>
</indexterm>

may produce, in a page layout output, and index entry of:

EJ25 engine
performance .................... 15

The page reference is applied to the innermost term (in the example, performance) only.

An example of a more complex index result is:

EJ25 engine
    dimensions .................... 72
    maintenance .................... 33
    performance .................... 15
    specifications .................... 49

In a hypertext document that supports multi-level indexes, the same example might be displayed as:

Screen capture showing a multi-level index in a Help system

You should avoid nesting index entries to more than three levels. In most cases, one or two levels will provide sufficient scope for a comprehensive and easy-to-navigate index.

If you have a number of secondary entries belonging to the same parent entry, such as EJ25 engine, performance and EJ25 engine, torque, the secondary entries can either be nested under the same parent entry, or structured separately.

In other words,

<indexterm>EJ25 engine
  <indexterm>performance</indexterm>
  <indexterm>torque</indexterm>
</indexterm>

is equivalent to

<indexterm>EJ25 engine
  <indexterm>performance</indexterm>
</indexterm>
<indexterm>EJ25 engine
  <indexterm>torque</indexterm>
</indexterm>