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.