Identifies the type of the related resource with an absolute URI
Can be the namespace URI of the root element:
http://www.w3.org/2000/10/XMLSchema
http://www.rddl.org/arcrole.htm#SOCAT
http://www.ascc.net/xml/schematron
http://www.xml.gr.jp/xmlns/relaxCore
http://www.w3.org/2000/01/rdf-schema#
Can be a MIME media type URI at http://www.isi.edu/in-notes/iana/assignments/media-types/
http://www.isi.edu/in-notes/iana/assignments/media-types/application/pdf
Can be a spec-defined URI:
http://www.rddl.org/arcrole.htm#DTD