Most Taggers don't make use of the extended functions
of the ID3 v2 Tag which can host a lot more information than you think.
The AllFrames Mp3 Tagger displays ALL available information from ALL
available Frames in a Mp3 tag. Even lots of information stored by other
companies etc.

You can store all sorts of additional information in user defined private
Frames such as ratings, properties, personal text, urls etc. And you can
read them!!