DITA topics are the basic units of DITA content and the basic units of reuse. Each topic contains a single subject. Topics might be of specific specialized information types, such as task, concept, or reference, or they might be generic, that is, without a specified information type.
<concept>, <task>, or <reference> Topics can be generic or more specialized; specialized topics represent more specific information types or semantic roles, for example, <concept>, <task>, <reference>, or <learningContent>.<topic> is the base topic type from which all other topic types are specialized. All topics have the same basic structure.