I assume he/she has it from here.
One user in another forum wrote here about FMPS_RATING, back at 2019-02-20:
The fmps_rating is already fully supported (read/write) by a lot of music players like Amarok, Clementine, Cantata (mpd client) Rhythmbox, Banshee etc.
I couldn't find a specification for it in a hurry.
2.2.1 All Rating Tags
• Ratings SHALL be a float value between 0.0 and 1.0, inclusive.
0.0 SHALL be the lowest possible rating; 1.0 is the highest
possible rating.• Ratings SHOULD only be rounded when necessary, in order to
increase cross-player compatibility.2.2.2 FMPS_Rating
• The canonical rating value in FMPS_Rating SHOULD be set
whenever a user rates the track. This is in addition to any
value stored for that particular user in FMPS_Rating_User. This
value is canonical because if a player does not support
multiple users (or if no user identifier is set), this is the
value that MUST be returned.• If a user removes ratings from a track in the media player's
database, the value of the FMPS_Rating tag SHOULD be cleared as
well.
From the specification starting at Line 235.