A new frame called TMOO aka "mood" was added to the ID3v2.4 standard. (
http://www.id3.org/id3v2.4.0-changes)
The still more common ID3v2.3 standard doesn't include any defined frame for a "Mood" tag. (
http://www.id3.org/id3v2.3.0)
Propably the first program that added a tag field for "Mood" was Musicmatch and its way to store the Mood field could be considered as the de facto standard a few years ago. MC uses that style for historical and compatibility reasons.
Here is a suggestion to the developers:
Read different Mood tags in the following order and apply the first found value to the Mood library field:- COMM, description: MusicMatch_Mood (the old style)
- TMOO if exists in ID3v2.4 tags
- TXXX, description: MOOD if exists in ID3v2.3 or ID3v2.4 tags
WriteIf the format is v.2.3:- COMM, description: MusicMatch_Mood (the old style)
and
- TXXX, description: MOOD
If the format is v.2.4:- COMM, description: MusicMatch_Mood (the old style)
and
- TMOO
and perhaps also (it would add compatibility with programs or devices that do not support TMOO, but can read TXXX tags):
- TXXX, description: MOOD
EDIT
On second thought, it might be better to give "COMM, MusicMatch_Mood" tag the lowest priority. Otherwise an externally added new TMOO or TXXX Mood tag would not be readable if the file already contains an old COMM Mood tag. The lowest priority would be enough for compatibility with old MC/MJ/Musicmatch tagged files.