In our approach to topic map schemas, all topics, occurrences, and associations are instances of classes. The classes themselves are expressed as topics. The class–instance relationship declared by the <instanceOf> element is in fact merely a syntactically privileged association class defined in the text of the XTM 1.0 specification. If we are looking at the class–instance relation from an ontology/taxonomy view, then there is a justifiable demand for a superclass–subclass relationship as well.
Our example contains some superclass–subclass relationships:
Company → customer, owner
Employee → team member
Team member → project manager
Technology → product, standard
The XTM specification predefines PSIs that specify both superclass–subclass ...