Hi, We've been discussing references, meaning things that are not defined locally, but you do want to provide additional information about them. For XML there has been a proposal to use id / idRef to denote something like that. Unfortunately it is not very easy to port that construct to RDF/OWL. The only way to express something like that in RDF/OWL is by using a relation. Fortunately a relation like that already exists in the form of rdfs:isDefinedBy. This states that an object is actually defined by another description, and conveniently provides the URL of that description to reference it. Could we perhaps use the isDefinedBy construct also in XML to make the reference definition somewhat more explicit? This would really help keeping the difference between the two syntaxes at a minimum also. Jeroen.