3.2.2.2 Purpose of DITA maps near the end says: Specialized maps can provide additional semantics beyond those of organization, linking, and indirection. For example, the subjectScheme map specialization adds the semantics of taxonomy and ontology definition. To say subjectScheme can do an ontology is a stretch. Ontologies can model any kind of data from soup to nuts to carburetors. A full ontology markup language is something like the W3C OWL2. That is perhaps possible with specializations but not subjectScheme itself. Being more simple than being a full ontology definition system is a strength in many cases. SubjectScheme seems to share a basis from SKOS which is very well suited for implementing a Thesaurus. So the words semantics of taxonomy and ontology definition could read semantics of taxonomy and thesaurus definition. I can put this comment in DitaWeb as well if you wish. cheers Jim