But just to be explicit about this, we are (deliberately) making the same mistake TEI made with @type of <name> and friends: the value of @type here does not describe the type of the element itself or the stuff inside the element, but rather categorizes the type of stuff that the elements inside refer to or describe. (That's why I liked @descirbes better, but if <xenoData> is going to be used for so much more than describing the source and describing the TEI file, that may not be a good idea.)
I'm convinced. I'll put it into att.typed and give it a private copy of @type with a suggested values include list of "source" and "transcription" (anyone come up with a better term?) and let Martin or anyone else add whatever others seem appropriate.
People can then poke at that.