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Presumably when the program sees a null in the middle of the comment it ignores the whole thing. I'm not sure what algorithm programs use for this.

In most programs you wouldn't know these extra COMM frames were there at all. I've seen content descriptors like iTunPGAP, iTunNORM, iTunSMPB, iTunes_CDDB_1, iTunes_CDDB_TrackNumber. ITunes adds several of these extra COMM frames for its own purposes. There is a null between the content descriptor and the actual value. Then any additional comments are in extra COMM frames which have a special content descriptor. The regular comment is in a frame called COMM (or COM in the earlier v2.2 standard). This is unlike most other tag types which can only occur once in a file. In ID3 a file can have multiple comment tags. Please correct me if I got anything wrong. I've written down what I found so far to help other people. Few people seem to understand it fully and most of them are on this board. I've done a lot of searching but there isn't much information around. The result is the tags often get mangled and you end up with garbage in the comments like strings of hexadecimal numbers. iTunes and mp3tag can handle them correctly, but some other programs get confused. It looks like iTunes adds these as multiple comment frames. These are for things like volume normalising (SoundCheck), gapless playback, CDDB, etc. I was confused by the special tags that iTunes adds to mp3 files. I'd like some help, please, from Florian and the experts here.