From the online help available at https://docs.mp3tag.de/customization/options/tags/mapping:
The problem is this: Winamp 5.666 is not able to read [or at last display] content from made up tag fields- apparently it has imprinted a limited list of processed fields. [They are listed and described under the "ATF Help" which clearly names their sources: "Provided by Winamp", "Provided by ML", "Provided by ML or input plugin", "Provided by input plugin"; and so apparently no provided-by-user fought came into the minds of its designers]
To bypass that issue I can create in Mp3tag my own tag field named YEAR-ORIGINAL or use the already existing "legit" tag field named ORIGYEAR. I would use it for storage of information of the first publication of a given song [and not merely a YEAR of an album on which it was released]. But if I do, then such data will not show up in Winamp, as both fields YEAR-ORIGINAL and ORIGYEAR are not listed by it as proper ones. So I thought I could use the DATE tag field- and so I checked the Winamp list. Unfortunately DATE was not on it. But then I remembered that in spite of that it is already mapped in Mp3tag. I checked my Mp3tag setting and there it was. And yet somehow it was also displayed in Winamp without a problem
So first two questions would be: is this a mistake on account of Winamp- they missed the DATE field and that is why it displays it? Or is it how mapping works [at least in Mp3tag]- it will not only substitute one field for another in Columns and Tag Panel [but not in Extended Tags] but also store mapped information in such way, that a third party software [like Winamp] will see YEAR instead of DATE? After all, my FLAC files under the the Extended Tags icon do not show me YEAR tag field but date
Next I made a test. I deleted mapping for DATE and created a new FLAC file without any data in its tag fields. And then I filled in YEAR with "0000" and saved it. As a result my Winamp displayed "0000" as a YEAR information
So now the questions are: can I just use YEAR field in FLACs instead of its DATE [and without mapping DATE to YEAR]? What is then the mapping needed for? To accommodate some other players than Winamp, which cannot be fooled into seeing DATE as YEAR?
And now I remembered my problems when I was searching for more lossless formats that would behave in the same way as FLACs and WavPacks [and MP3s]. Back then I had to dismiss them on the account of various issues. Most important here I had to deem the WMA format as unusable for me because of the problem with a single dot in the TRACK tag field [/t/17714/1]. But in the light of those above findings of mine, it seems that I could just create for example YEAR-UNIVERSAL field, map it in Mp3tag as the YEAR and no longer have issues with a >>.<< in TRACK among the WMA format. Because there will be no longer a problem with putting such data in a made up by me field of YEAR-UNIVERSAL [although there still could be some other issues]. And this workaround also shows potential for dealing with other format with issues of that kind
So the last questions would be: is such mapping even possible for WMA at all? And if yes, then how do I do that? What do I put in the Tools > Options > Tags > Mapping > Tag? Both "VorbisComment" and "ID3v2" do not work, while my guesses at using "Advanced Systems Format", "AdvancedSystemsFormat" and "ASF" are not accepted [remembered] at all