in id3 the frame for "contentgroup" is TIT1
mp3tag says this:
http://help.mp3tag.de/main_tags.html
i don't use that frame, or the "category" field in winamp, so i don't know if that field is the TIT1 frame or not, or what functional purpose the frame is supposed to serve, if any.
i don't know much about m4a, but flac uses vorbis tags which are a weak standard.
basically, what you seem to be after, is for winamp to "map" the "category field" to the "correct" spots/nomenclature in the 3 formats.
i don't know what that would be, but you need to cite documentation showing why or what uses it the way whatever uses it.
i can't even explain why winamp would call TIT1 content group: "category."