There's also been discussion of POPM in WinAmp and how the rating scheme differs from WMP:
http://web.archive.org/save/forums.winamp.com/showthread.php?s=&threadid=315444

I believe Winamp just uses a rating of 1 to 5, while WMP uses 1 to 255.

Another one on SuperUser re: WinAmp and POPM
http://web.archive.org/save/superuser.com/questions/474268/how-does-winamp-save-the-rating-of-songs

There might be some use in saving the values so that you get predictive results in the player you use, and there might be some presets to do so.

But the mappings in that ticket are just entirely insane. The day we have a mapping that says 28 = 1 star, 29 = 2 stars and 30 = 1 star I will just stop using Picard
And I am sure you have solved that better in Jaikoz.

Update: Just because they were mentioned in the ticket above, the Banshee player has documented this pretty nicely in their code (and they actually use a sensible scheme): https://git.gnome.org/browse/banshee/tree/src/Core/Banshee.Core/Banshee.Streaming/StreamRatingTagger.cs

Im not sure they are that obscure, although the linear mappings Picard uses may be sensible Picard is not a player and the same values will be rendered differently by Itunes and Windows Media Player ( we did add an option to Jaikoz to allow users to specify their player so that we can do the mapping correctly from/to Jaikoz/Player they could choose from mappings for Itunes , WMP and Media Monkey)

The linked ticket has a lot of strange suggestions on obscure mapping tables for ratings (which IMHO are all nonsense, the linear mapping we currently have in Picard actually makes sense).

But the main point is that POPM tags in ID3 require an "e-mail" to be set (to identify for which user the rating actually is). Picard does support setting this, and if you want to set the ratings for display in WMP you can just set this e-mail to "Windows Media Player 9 Series".

However, Picard might provide some kind of UI support for this use case (e.g. a dropdown of typical values, or just a checkbox "Write WMP compatible ratings to ID3"). I have not researched yet how relevant this still is and whether "Windows Media Player 9 Series" still applies to latest versions of WMP.