Please add fields to mp3tag in line with iTunes 12.3/Get Info/Sorting box

It seems necessary to add some fields to mp3tag because iTunes 12 has altered the way it handles sorting of fields in Get Info/Sorting tab.

Even if you choose sort Albums by Title in iTunes, the result in iTunes can be anything but sort Albums by Title.
This turns out to be because iTunes 12 has added a Sorting tab to Get Info for file information.
In particular, Name, Album, Album Artist, Artist and Composer can be sorted by iTunes 12 as something quite different to Name, Album, Album Artist, Artist and Composer as input by the user.
The 'something quite different' is probably fields taken from the original tags, rather than from the user created tags.
I like sorting Album by Album Title, but because iTunes 12 has a Sorting tab with 'Album/Sort as', the order that ends up in your iPhone Music is not sorted by the user defined Album Title.
An attachment shows an example of how Name, Album, Album Artist, Artist and Composer fields can end up sorted quite differently to the user defined fields.


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What about the (already existing) fields
ALBUMARTISTSORT, ARTISTSORT, ALBUMSORT, COMPOSERSORT, TITLESORT

I have looked at mp3tag, and I don't know what you mean by these sort fields- I can't find anything that resembles these sort fields.

Please explain how you find them in mp3tag.

See the help: http://help.mp3tag.de/main_tags.html
They are just ordinary fields like ARTIST, TITLE and so on.
You can add them to the Tag Panel, the columns, enter them as new fields in the Extended Tags dialogue and treat them with actions.