CXTM is defined in ISO 13250-4, that is, part 4 of the Topic Maps standard. It's an XML syntax for topic maps, but not one intended for interchange. (XTM takes care of that.) Instead, CXTM is meant to be used for software testing.
CXTM is currently a Committee Draft, but went out for ballot as a Final Committee Draft on May 15th 2008. The ballot results are not yet in.
More information:
The following CXTM implementations are currently known:
The current editors are Lars Marius Garshol, professor Jaeho Lee, and Motomu Naito. Feedback can be sent to them or to the topicmapmail mailing list.