says "ID3 is a de facto standard for metadata in MP3 files; no standardization body was involved in its creation nor has such an organization given it a formal approval status. It competes with the APE tag in this arena."
Re POPM, ID3 says "Popularimeter
: The purpose of this frame is to specify how good an audio file is." (i.e., rating)MP3Tag
uses POPM as the main tag in additon to non-ID3 tags like 'Rating WMP' and 'Rating MM' for Windows Media Player and Media Monkey respectively.
I would suggest MusicBee is doing the right thing using POPM when you check the 'save Mp3 tags as ID3v2.3
If you really want to keep Rating WPM, you could possibly use MusicBee's 'advanced search and replace' option and custom tags but that is way beyond my skill levels!