Extensibility

RDF encourages ad-hoc conventions/extensions:

rdf:type
rdfs:subClassOf
rdfs:subPropertyOf

      <wsse:SecurityToken wsp:Preference="100">
        <rdf:type rdf:resource="http://signon.example/#SingleSignonToken"/>
        <wsp:Usage rdf:resource="http://www.w3.org/2004/08/20-ws-pol-pos/ns#Required"/>
        <wsse:tokenType>wsse:Kerberosv5TGT</wsse:tokenType>
        <ss:issuer rdf:resource="http://signon.example/agents#intra1"/>
      </wsse:SecurityToken>