This is an unique identifier defined in the W3C Candidate Recommendation of the XForms 1.0 Specification. This W3C Candidate Recommendation can be found http://www.w3.org/TR/xforms/

For more information about XForms, please refer to the W3C XForms overview.
For more information about XML, please refer to The Extensible Markup Language (XML) 1.0 specification.
For more information about XML namespaces, please refer to the Namespaces in XML specification.