The following fields are available for ID3v2 and MP4 tags
MOVEMENTNAME
MOVEMENT
MOVEMENTTOTAL
The WORK field is only available for MP4 tags. For ID3v2, what is filled in the Work Title in iTunes is currently displayed as CONTENTGROUP in Mp3tag. The developers at Apple decided to reuse this field which was previously used to store what is filled in Grouping. This information seems now to be stored in the iTunes database only and not in the ID3v2 tag. For MP4, what is filled in Grouping is still presented under CONTENTGROUP in Mp3tag.
It's unfortunate that they've created this inconsistency here. If someone from Apple is reading this, I'll happily discuss a more robust approach.