The list I included of fields actually came directly from MP3Tag's list of "Extended Fields" in the user interface (Customize Columns > Field drop down > Extended Fields).
I'm not sure it's accurate to say the POPULARIMETER field is "standard", since the support for said "standard" is basically non-existent across players and tag editors. 
Searching Google for "POPULARIMETER id3 tag field" and "what mp3 players use "POPM"" and similar yields almost no useful references other that people discussing their confusion/frustration with trying to sort through this issue. Also, even if you consider the field a "standard", the way players do rating and store the ratings varies widely...which means the "standard" really isn't much of a standard. Note also that some players store stuff other than the rating (e.g. evidently MM with a rating of 5 stars, writes "no@email|255|0"). What a mess. 
It would be great to put together a table showing various players/MP3 tools and which field(s) they store/read ratings from and the format the ratings are written in and/or expected to be read in.
Ideally, MP3Tag could detect which field(s) are in use and display the rating as a number or star rating.
There's currently an option in MP3Tag under Options > Tag > Advanced that you can turn on/off called "Always display POPULARIMETER values for ID3V2 ratings. I've no idea what that REALLY does since the help documentation is vague about what happens if it's turned off... It also doesn't explain where or when the value would be displayed. Can anyone shed light on that? I'm gonna experiment a bit more.
See Also:
- POPULARIMETER tag
- https://www.reddit.com/r/linux/comments/tx7tz1/there_seems_to_be_no_linux_music_player_that/
- Note: http://www.id3.org is currently down...what's the story with that?