I am in the process of adding my whole audiobook & audio drama collection to audiobookshelf (self-hosted audiobook server). During this process I found out that the built-in series detection of abs does not really work for my folder structure, which is why I used mp3tag to correctly tag all my files. Everything worked fine, except for series. abs expects it to be in the "grouping" field ([Bug]: Series and series-part do not get embedded when you write meta data to files. · Issue #3547 · advplyr/audiobookshelf · GitHub). So I used this in mp3tag and proceeded to tag the books. But, alas, it did not work.
By trial and error (and the usage of another id3 tagger, kid3) I found out that audiobookshelf expects the grouping field to be with a TXXX frame, so a custom user-defined field.
How could add such a field in mp3tag? I know that there is the 'proper' grouping field (GRP1) available but this is apparently not ready by audiobookshelf.
Thanks!
Hello Cantello, your request is very interesting
Can you be more specific about the name of the tags you want to use…
The tag that is supposed to be used is "GROUPING" but the tag that audiobookshelf actually parses appears to be "TXXX:grouping". I do not know exactly where the difference is, but in kid3 I was able to have two different tags, one is TXXX, the other one is GROUPING.
See the mapping for GROUPING:
If there is a standard field, then MP3tag will not allow you to write it as a user-defined field.
AFAI see it, GROUPING is not available as standard field for MP4 files - so there you would get a user-defined field with the name GROUPING.
That's what I thought as well but the above screenshots are from kid3 and the first one is directly from mp3tag and the second one is what audiobookshelf writes to the tags, so mp3tag produces the 'regular' tag and audiobookshelf the user-defined one.
So my question remains, is it somehow possible to recreate the TXXX tag with mp3tag? So far, I have been unsuccessful.
Yes - but not for standard tag fields.
If you edit such a file as created by kid3 then the TXXX field will be transformed into a standard field. (for files where that is valid).
Thanks, that's what I thought. So audiobookshelf will not read the standard (even if only by practice, it's not part of the 'official' id3v2.3 definition) GRP1 tags and mp3tag will not write the older TXXX tags. What a dilemma... 
Mp3Tag can write multiple value tags. But with GROUPING each of these tags will be a GRP1 and not a TXXX.
Hello Poster, I am trying to understand the result of the TAG GROUPING, I do not know this TAG…
What is the perfect result of writing the TAG GROUPING in a file... With or without TXXX
Please if possible, write or post a photo of a perfect writing result, for this TAG...
This field will enable you to link several tracks by giving them the same grouping value.
It was invented by Apple. The result depends of software that makes use of this tag.
Correct, it was an attempt to make it easier primarily for cataloguing Classical music in iTunes.
And unfortunately, this is now what audiobookshelf uses for the book count in a series. Maybe it is better for .m4b files, which are apparently widely used for audiobooks, but for me, this sucks. I don't want to use audiobookshelf to index all episodes and I don't want to use text sidecar files, I just wanted to use mp3tag to add the correct tags and utilise its rather powerful scripting to extract sequence numbers from the album tags.
Well, I will probably just leave the series unsequenced.
Thanks to all for trying to clear all up! 
Thanks for the info, Poster...
Too bad this TAG GROUPING cannot be used for Cantello's project...
?
The tag field is already available.
Se the already linked mapping:
I corrected my text...
Thanks ohrenkino
I see the problem on the players side as it fails to interpret a standard tag field.
You're right ohrenkino, as you know I had the same problem with my BSI Simian tags and thanks to LyricsLover, my tags work 100% in an unconventional way, I admit it, but it's great to work with Mp3tag, It's faster, I couldn't do without it...
This is my opinion, I believe it is possible to make this non-compliant TAG GROUPING (GRP1) work with Mp3tag... There must be special programming...
I see that for several years, TAGS programmers have been respecting the standards in force…
According to my research, in 1998 a new specification called ID3v2 was created, I believe some TAGS were developed over the years...
At the beginning of the creation of tags, I imagine that the IT companies created tags which were not compliant in the industry…
BSI Simian are among the first to make a TAGS computer program for radios
Their first edition was made in 2001, I imagine that searching for information on the web at that time, for the TAGS standard, was not as easy as it is today...
If you want to use Mp3Tag for tagging anyway the only way seems to be to make all your tagging-works with mp3tag and after that use another piece of software, which is able to write an additional TXXX-frame for grouping. But when you've done your work you should not touch and save the file with mp3tag again.
Meanwhile there is also the V2.4 standard ...
Everything would be OK, if the standard would be implemented completely.
I do see that some tag fields are harder to interpret than others as they have a complicated structure. But in this case it is a straightforward field following the pattern of a lot of other fields.
What it makes even worse is the use of a user-defined field for a standard purpose but leaving out the implementation of that standard field.
Musicbrainz uses a number of user-defned fields but has added the prefix "Musicbrainz" to the names so that any collision with a future standard field is not very likely.
It would have been a good idea by the developers of audiobookshelf to do something similar in their program, e.g. call the field "audiobookshelf grouping".
But all this is just idle talk - we have to cope with the realities and there the dilemma cannot be solved without ending in a dead end.