This is an XML namespace defined in the WebCGM 2.0 Specification. WebCGM 2.0 is a collaboration of OASIS and W3C. It is [or presently, "will be" is accurate] both an OASIS Standard and a W3C Recommendation, which are identical in all technical aspects, and differ only in the format and presentation style of the respective organizations.
The latest OASIS-published version of WebCGM 2.0 can be found at http://docs.oasis-open.org/webcgm/v2.0/.
The latest W3C-published version of the WebCGM 2.0 can be found at http://www.w3.org/TR/WebCGM20/.
For extensive information about CGM and WebCGM, please refer to CGM Open home page, and to the home page of the OASIS CGM Open WebCGM TC. 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.