Urn is a subclass of Identifier, as are about 30 other classes which are identifier types. Urn is just another identifier type that might identify a resource, in the same manner as an ISBN might identify a resource. A Urn is in fact a URI, but that’s irrelevant. It is not an HTTP URI. HTTP URIs, and only HTTP URIs, are used to identify resources that are subjects or objects of RDF statements.