Mac (and Windows) feature suggestion ...
Would be nice to have a way of entering a custom description that could popup when one hovers over a Tag Panel or File List field.
In my case, for example (see screen capture), I renamed the field names in the Tag Panel to include the (ID3vx.x) name ... would be nice to be able to move that over to a "description" that pops up when I hover over the field. That way I could add more details, ex: APE, Itunes, etc.)
The screen capture is for Windows but the same would apply for Mac. Would also be nice to see the "Description" you added to the Action Groups make its way into MP3TAG for Windows.