Subject scheme maps and their usage
Subject scheme maps can be used to define controlled values and subject definitions. The controlled values can be bound to attributes, as well as element and attribute pairs. The subject definitions can contain metadata and provide links to more detailed information; they can be used to classify content and provide semantics that can be used in taxonomies and ontologies.
A DITA map can reference a subject scheme map by using a <mapref> element. Processors also MAY provide parameters by which subject scheme maps are referenced.