Specify how or if you want the Grouping field to be populated, you can specify the source and the maximum number of genres you want to add. Genres are added to a single field allowing all genres to be search for within one field, the grouping field is often used in iTunes as a way of supporting multiple genres for a song.
Grouping: Replace only if empty, add values, replace values or do not modify field
From: Use Discogs Style, Discogs Genre or both
Max no of Genres, songs are often associated with multiple styles this specifies the maximum number of values to add
Warning:this field was originally for storing Classical music works and since iTunes 12.5.3 is now used for this again for MP3 and AIF formats, if you have Classical Music in your collection you should think carefully before using this option for grouping genres.