I can see the %ITUNESCOMPILATION% tag in the Foobar2000's properties but not in MP3Tag's Extended Tags; it just shows up as COMPILATION.
I'm guessing the iTunes Comp tag is left over from when I used iTunes years ago. So, when I add a 1 to the compilation field in MP3Tag, is it actually writing to the left over ITUNESCOMPILATION tag?
So, what is the field literally called? I'm confused about whether Foobar2000 is representing the COMPILATION field as ITUNESCOMPILATION or is MP3Tag doing the reverse?
BTW:
This "translation"-mapping was already mentioned in the first answer from @ohrenkino.
Nobody can remember 4 letter abbreviations like TCMP for COMPILATION.
Therefore every tagging tool is using slighty different names but all write the data back into the same TCMP field in this example and for MP3 files.
I see. I think this is the part that was confusing me, I thought TCMP was a kind of tag or a remapping scheme, or something. Thanks for the clarification.
In MP3tag COMPILATION is the reserved name for the field as described earlier. So it is not possible to create a user-defined field with the same name.
What would be the purpose of this non-standard field?
This is only possible if you create a so called user-defined field like
MYCOMPILATION
or
COMPILATIONMRMECH
or something that is clearly different from the official
COMPILATION
The problem with user-defined fields is that they are not supported by most third-party software, like smartphone or car players.