Interesting; I just discovered and used this tool a month ago. For years I've been wanting to migrate my few hundred remaining MP3 files from iTunes/Music to just store them on the file system but iTunes DB stores play count and star rating in their DB, not in the file itself.

Turns out that there's no standard tag for this, but the most common (from what I searched was POPULARIMETER, where you store it in EMAIL|RATING|PLAYCOUNTER format. Email is a string (optional), rating is an int 0-255※, and playcounter as an integer.

So I wrote a Python script that would read the file on disk, match it with the entry in iTunes DB, populate POPULARIMETER, then verify everything was set correctly in MP3tag.

This took a few hours to do, mostly due to discovering how to do it and verifying everything worked correctly. Unfortunately the MP3 players I use now (VLC and mpv) don't support updating the POPULARIMETER field, so it'll be left as an unchanging relic from my iTunes days.

※ - 1 star → 0.2x255 = 51, 2 star → 0.4x255 = 102, etc