I just made some tests with the new "Serato DJ Pro v4.0" and added field content in Serato for 4 different music formats: FLAC, M4A, MP3 and WAV
and for Remixer, Label, Grouping and the mentioned Rating.
The above fields (and some more not so obvious) are filled from Serato in these metadata fields:
As @djbeware wrote, Serato is using POPULARIMETER for the star-rating in MP3 and WAV.
For MP3 and WAV Serato writes 1 (1-star), 64 (2-star), 128 (3-star), 196 (4-star) and 255 (5-star) into the POPULARIMETER field.
For FLAC the field RATING is used and for M4a the field RATE.
The POPULARIMETER content is interchangeable. To re-read the externally changed values from Mp3tag, simply right-click in Serato and select 'Re-Scan File Info'.
For MP3, the Serato fields Remixer, Label, Grouping and Rating are written into MIXARTIST, PUBLISHER, CONTENTGROUP and the mentioned POPULARIMETER.