[RFC3688]
defines an IANA maintained registry of XML documents used within IETF
protocols. These documents are divided into classes: XML Namespaces,
Document Type Declarations, Schemas, and RDF Schemas. Individual documents
are then registered within each class. The class and document names
are combined to form a name which is then used within the 'urn:ietf:params:xml'
portion of the IANA maintained URN space for protocol parameters.
The following is the list of classes:
publicid
ns
schema
rdfschema