https://wiki.asterisk.org/wiki/display/AST/Asterisk+13+Application_CELGenUserEvent
There is a usage example in Reporting and statistics — XiVO Solutions documentation However the interpretation of the extra field is dependent on the particular software product, and there is no requirement that it contain delimiter characters.
There is also an example in a recent version of Asterisk the Definitive Guide, but not one for which the text is freely available online.