I used the scripting function $Set() to save data to the field "Grouping" in order to populate the iTunes attribute "Grouping". Code used: $Set(grouping,Picard Grouping Attribute)
The string "Picard Grouping Attribute" does not appear in iTunes. Afterwards I typed "iTunes Grouping Attribute" into the grouping field in iTunes. The Tag-Editor mp3tag shows that Picard populates the attribute "Contentgroup", iTunes uses "Grouping" instead.
I expected Picard to fill the Grouping-Attribute. If I'm not wrong, this should be the case according to https://picard.musicbrainz.org/docs/mappings/#cite_note-lastfmplugin-2.
Please find some screenshots attached.