A content metamodel is formally described by an XML-schema document. When in doubt about how to express a content model or understand the full capabilities of the content metamodel, interrogate the XML schema as it provides the definitive description of the content metamodel.
The target namespace of the content metamodel XML schema is
www.alfresco.org/model/dictionary/1.0
Note: You can locate the latest content metamodel XSD schema at:
http://svn.alfresco.com/repos/alfresco-open-mirror/alfresco/HEAD/root/projects/repository/config/alfresco/model/modelSchema.xsd