In the rest of the Primer, the term vocabulary will be used when referring to a set of URIrefs defined for some specific purpose, such as the set of URIrefs defined by RDF for its own use, or the set of URIrefs defined by example.org to identify its employees. The term namespace will be used only when referring specifically to the syntactic concept of an XML namespace (or in describing the URI assigned to a prefix in a QName).

URIrefs from different vocabularies can be freely mixed in RDF graphs.