The usual advice is to store CD label data in the PUBLISHER tag (or the ORGANIZATION tag for FLAC). More than 20 years ago the ID3 tag TPUB was defined as "the name of the label or publisher". In my opinion that "or" has caused confusion and sloppy data entry ever since. A publisher is a business organization but its CD or record labels are product brand names. Those names may or may not be the same and confusing them is common.
The confusion is magnified by the fact that one publisher may own many CD labels. Perhaps that is why iTunes wisely provides separate label (©lab) and publisher (©pub) tags. I very much prefer this approach.
This spring I began using dBpoweramp 17.6 to rip and tag my CD collection to FLAC and/or M4A. After ripping I edit the tags in Mp3tag. However, in the Publisher tag I found grossly inconsistent values from the Internet databases. Sometimes the tag showed the name of the parent company, sometimes the CD label, sometimes both, and sometimes both plus the catalog number or other data, all in one tag! I have even seen Publisher used to store the name of the song publisher on each track. In short, Publisher content from the Internet is often a mess, even when it conforms to that ancient ID3 spec.
None of this is the fault of dBpoweramp, which is a remarkable application, by the way. It simply passes on content from the Internet.
To get a grip on this problem I created a custom tag LABEL in Mp3tag. This tag holds the CD label name and only that name. Then I added the following mapping line in Options for my M4A files:
MP4 > ©lab > LABEL
My FLAC files do not need mapping because Mp3tag already recognizes LABEL tags in FLAC files.
Before ripping a CD in dBpoweramp, I select all tracks and type in the label data and the catalog number (except when the latter has been pulled from the Internet automatically).
For my existing files I simply select all tracks for the album in Mp3tag and type or paste the label name and the catalog number into the tag panel boxes. Often I then delete the Publisher tags.
These extra steps add a little time but in the end I find it worthwhile to have consistent data. For example, it is sometimes enlightening to sort my CD rips by Label in Mp3tag. A meaningful sort on a raw Publisher column is often impossible because of the mix of data there. With a dedicated Label column the sort is easy and accurate.
Given how well my custom Label tag works, I don't know that adding internal Mp3tag support for it would offer me much. But I am open to the opinions of others on that. I'm also curious about how others deal with the mix of content that so often appears in Publisher tags.