more formal refence to existing IVOA vocabularies

[pdowler]

Plane.observable.ucd -> UCD1+ spec at https://www.ivoa.net/documents/UCD1+/
Plane.dataProductType -> http://www.ivoa.net/rdf/product-type aka ObsCore datya product type
Artifact.productType -> http://www.ivoa.net/rdf/datalink/core aka DataLink semantics

[pahjbo]

This is what the "SemanticConcept" part of VO-DML is intended for - VO-DML allows this to be a bit more general than just the vocabulary lists that have proved to be the most useful, and so I think that it might be useful if the VO-DML standard gave concrete guidance in exactly how to use SemanticConcept for the standard RDF vocabularies.

UCD could be defined as a "primitive" type in the base IVOA model....

[pdowler]

I will look at SemanticConcept more closely... thnx