Work/Grouping fields (iTunes 12.5)

I had been using the GROUPING field in iTunes for format, source, year, country, genre and mood labels - with many smart playlists generated using this information. I use mp3tag to process mp3 files prior to importing them into iTunes, adding basic info to all tracks on an album in mp3tag and then detailed info to tracks individually inside iTunes whilst listening to the music. I just found out that any info added to the GROUPING field within iTunes 12.5 is no longer embedded in the mp3 file itself, but saved only in the iTunes library (not good). Apparently the new WORK field in iTunes is where I need to store the information previously held in the GROUPING field, to ensure that any data I enter in iTunes is written to the mp3 tag.

Would somebody please tell me how to create an ACTION that will copy all of the information currently saved in the GROUPING field into the WORK field in mp3tag. As far as I can see, they both link to the TIT1 field. Thanks

See the release note on Version 2.80c
Mp3tag v2.80 released
which tells you the field names in MP3tag.
Copying one field into another could also be achieved easily with the function
Convert>Tag-Tag

I was confused because of the re-purposing of the CONTENTGROUP tag from Grouping (old) and Work (new). It seems that all info has been copied by default. Using the Work tag doesn't feel right though, so I look forward to an update to mp3tag featuring a dedicated grouping tag (GRP1?).

Here you see some of the discussion: work name, name and movement
It could be that you wait in vain as iTunes seems to store that only in its database.

Since iTunes release 12.5.4.42, it read & writes iTunes Grouping to a new id3 frame "GRP1", while the TIT1 frame is dedicated to the Classical "Work" field. If and when Mp3tag adds support for the GRP1 frame then we will have direct access to iTunes Grouping again.
There is a yet unanswered question on this forum on whether there are plans to support the GRP1 frame in Mp3tag. It seems logical to eventually support it, although Apple did breach the id3 standard by making up their own frame instead of using a TXXX frame, but the truth is no one really follows the id3 standard to the letter anyway, common convention has been more the rule rather than the rules themselves.
Support for new iTunes Grouping id3 frame "GRP1"?

Without actual support for the GRP1 iTunes Grouping frame, one workaround is to use a hex editor to batch edit all mp3 files and change "TIT1" to "GRP1", then reimport into iTunes. The first time you do this you will have to clear out WorK in iTunes Get Info to clear the Work stuff out of the iTunes library, but it should stay out after that. Also need to be using a copy of your files exposed to iTunes, keep your originals that Mp3tag references elsewhere. A hex editor can corrupt your files if you don't know what you're doing, proceed at your own risk. That said, I tried it and it worked for me.

I've added support for the iTunes-specific ID3v2 grouping field (GRP1) as GROUPING with the current development build Mp3tag v2.80d.

I've decided against any automatic conversion, but instead for a dedicated tag field. If you have any grouping values stored at CONTENTGROUP which you want to use with the new GROUPING field, you can use the "Tag – Tag" converter or an "Format value" action to copy the values.

Kind regards
– Florian

Thank you. Downloading now.