m4a files with hidden tags?


When I convert m4a to mp3 files with iTunes 7.2 I noticed there seems to be a sort of hidden USER tag inside the mp3. I can't see it with mp3tag or Winamp. But when I use Gspot it says under the section User data/Metadata: [USER] 2Mh)2u. I have 4 such tags, each with other data.
Even after deleting all tags with mp3tag there is still 1 left.
Other mp3's made with lame don't have those USER tags.
I noticed that a m4a header also contains lots of other stuff not visible in mp3tag like the iTunes accountname, .... (I made it visible with Faad the m4a encoder http://www.rarewares.org/aac-decoders.php)
What are these USER tags for?


Sorry, I really don't know and never seen such a tag.