It sees made up fields; at least those made up in Mp3tag
And it allows for putting whatever value you desire to even for example the GENRE tag, which was intended to be used with pre-defined values. Although there are some rare exception to what kind of data you can put in tag fields; but that is not the fault of Mp3tag, but of the given file format:
A] APE: /t/17672/1, http://forums.winamp.com/showthread.php?t=398515
B] M4A: /t/17672/1, http://forums.winamp.com/showthread.php?t=398515
C] WMA: /t/17714/1