Comment 1 for bug 1773683

http://www.jthink.net/jaikozforum/posts/list/19698.page

"So actually for ID3 formats the old grouping field is actually TIT1, and this was used by users to store things like genres. However when iTunes added support for classical music work and movements they decided to use TIT1 to store the work, and created a new field called GRP1 that mapped to their Grouping field.

So we now have the situation that most tools use TIT1 (also known as Content Group Description) for grouping, but iTunes uses it for Classical works and uses GRP1 for grouping, this affects Mp3 and Aif, there is not a problem with other formats."

Also here:
http://blog.jthink.net/2016/12/itunes-add-new-grouping-field-but.html