In DITA, a topic is the basic unit of authoring and reuse. All DITA topics have the same basic structure: a title and, optionally, a body of content.
DITA topics consist of content units that can be as generic as sets of paragraphs and unordered lists or as specific as sets of instructional steps in a procedure or cautions to be considered before a procedure is performed. Content units in DITA are expressed using XML elements and can be conditionally processed using metadata attributes.
Classically, a DITA topic is a titled unit of information that can be understood in isolation and used in multiple contexts. It should be short enough to address a single subject or answer a single question but long enough to make sense on its own and be authored as a self-contained unit. However, DITA topics also can be less self-contained units of information, such as topics that contain only titles and short descriptions and serve primarily to organize subtopics or links or topics that are designed to be nested for the purposes of information management, authoring convenience, or interchange.
DITA topics are used by reference from DITA maps. DITA maps enable topics to be organized in a hierarchy for publication. Large units of content, such as complex reference documents or book chapters, are created by nesting topic references in a DITA map. The same set of DITA topics can be used in any number of maps.
DITA topics also can be used and published individually; for example, one can represent an entire deliverable as a single DITA document that consists of a root topic and nested topics. This strategy can accommodate the migration of legacy content that is not topic-oriented; it also can accommodate information that is not meaningful outside the context of a parent topic. However, the power of DITA is most fully realized by storing each DITA topic in a separate XML document and using DITA maps to organize how topics are combined for delivery. This enables a clear separation between how topics are authored and stored and how topics are organized for delivery.