I have not seen such a document, though I haven't searched the web for it.
I don't know if mp3 and AAC/mp4 tagging support different tags. There are different versions of tags even within one system. I am pretty sure BPM is a part of of the tagging system for mp3s at least. I know I have added files and seen data appear in BPM.
There are, of course, also items where the data are stored only in iTunes, such as ratings. Then on top of that, some data are reputedly only stored in the .itl version of the library file and not the .xml version.
BPM is definitely stored in the MP3 file, and will be there if you copy the file and use it somewhere else.
Like Limnos, I have not see a nice summary, but here are the basics:
Identifying information about the song, such as Artist, Title, Year, etc are stored as tags in the file for most audio types. The exception is WAV files, which have limited tag capability.
Library-specific information, such as Date Added, Last Played, Play Count, etc, are stored in the library, not in the file.
And then there are some complicated things:
iTunes puts the Rating in the library, even though the tag is capable of storing it (and other players, such as WMP, use the rating tag).
Artwork is in the song file if it was added by pasting in using Get Info, but is in the Artwork folders if it was added using the Get Album Artwork command.
Good stuff ed2345 - this helps to me understand a bit more.
my problem specifically is with quick time video files.
I have moved my video folder around a bit - and when I add these files to my itunes libraray Artist doesn't come in on about 80 'older' video files - I can type artist info in in itunes but if I trash that and re-add those same files come in again with Blank artist info. - should I convert those quicktime movies to say mp4 newer codec ?
I think what is happening when you edit in iTunes is it creates labels it writes to the library file but these are not embedded in the actual file. The same thing can happen with artwork fetched from the iTunes Store but not actually embedded in the files (but I do know a solution for artwork).
Theoretically WAV format is the only common format that is supposed to present problems with embedding tags. In practice I have found issues with iTunes and tagging with pretty much all other formats (including .mov, .aiff and .mp4) except mp3 in situations. Sometimes it works, but frequently it does not. mp3 is the only format I have found to reliably allow editing and transfer with both iTunes and other applications.