I truly love the new custom tag mapping ability of Mp3tag 2.45. It works perfectly. Really, far better than I would have expected, the seamless nature of it is extraordinary. From saved actions to column values and the Tag Panel, there's zero confusion once you've set up the mappings to your liking.
But I have a couple of suggestions for the user interface in options. I apologize for not bringing this up prior to the 2.45 release.
The two columns should be reversed in their order. The names you're mapping should be on the left and what you're mapping them to should be on the right. People think from left to right, as in:
TRACK -> TRACKNUMBER
The column headers have almost no meaning to the end user. They probably mean something to the programmer, but that's of little practical use. I would suggest (after swapping the column aorder) something more understandable:
Field Name & Map To
There's some confusion when mapping to a field name that Mp3tag also maps internally, as in the many mapping for ID3v2 frames. The confusion stems from the included VorbisComment mappings, which many users may use as examples. These fields are not mapped again, while any ID3v2 mappings get mapped twice. With the VorbisComment mappings, it puts the Mp3tag internal field name in one column, while it's the opposite with ID3v2 mappings. I'd like to suggest that color highlighting any internally mapped field names would be a good idea.
Particularly because of #3, a dedicated Help page for User-defined mappings would be welcome, explaining how the process works and how names may be mapped twice by Mp3tag. Give additional examples, particularly for mapping ID3v2 names. IMO, this should be a separate page from the existing Mappings help page, which is already quite long. Provide links between the two Help pages.
As I read it, the current Help regarding User-defined mappings is wrong.
Given the pre-defined VorbisComment mappings, the "Source" is not the Mp3tag internal field name, but the name read and written in the tag.