“Occurrence” in topic maps is currently overloaded. Seriously overloaded.
In one sense, “occurrence” is used as it is in a bibliographic reference. That is that subject X “occurs” at volume Y, page Z. A reader expects to find the subject in question at that location.
In the overloaded sense, “occurrence” is used to mean some additional property of a subject.
To me the semantics of “occurrence” weigh against using it for any property associated with a subject.
That has been the definition used in topic maps for a very long time but that to me simply ripens it for correction.
Occurrence should be used only for instances of a subject that are located outside of a topic map.
A property element should be allowed for any topic, name, occurrence or association. Every property should have a type attribute.
It is a property of the subject represented by the construct where it appears.
Previously authored topic maps will continue to be valid since as of yet there are no processors that could validate the use of “occurrence” either in the new or old sense of the term.
Older topic map software will not be able to process newer topic maps but unless topic maps change and evolve (even COBOL has), they will die.