I use the sorting tags a lot in iTunes, e.g., to have artists sorted by last name instead of first. I'd love to be able to add those tags in mp3tag, rather than doing it in iTunes. If there's a way to create custom fields, maybe I'm missing it.
Anyway, just a suggestion/request.
-Mike
These tags are already supported. If you have a file with these tags you can see them with "View > Extended Tags..."
Just a clarification, can you only see them via extended fields if they're already in use (there's already info entered, I mean), or can you also create them via extended fields?
You can create them or use them in the columns like any other tag. You just need to know the names.
There's also a list in the help file [F1] "Names and mapping of ID3v2/WMA tag fields" but it is not complete.
I installed and began to use iTunes about a month ago and was going to post this to the forums, but they were locked. These fields need to be added to the tag mapping help page.
Here's a quick list:
ALBUMSORTORDER
ARTISTSORTORDER
BANDSORTORDER
COMPOSERSORTORDER
TITLESORTORDER
I played around with this a little bit tonight, testing Mp3tag's behavior. I think the first three are supported, but COMPOSERSORTORDER (iTunes' "Sort Composer", TSOC in id3v2) is not. I'm not sure about TITLESORTORDER (iTunes' "Sort Name", TSOT id3v2).
I'm finding Mp3tag nearly impossible to use these days with more than one tagging system. Adding, for instance, an ARISTSORTORDER field to a FLAC file is worthless. Something needs to be done. This tool obviously evolved out of something used only to tag MP3 files, but it hast not progressed very far beyond that and severely falls short of offering a convenient way to define Action Groups that can work with files of varying CODECS.
ARTISTSORTORDER is not worthless in my FLAC files. It is useful with my player and also when I convert my FLAC's to MP3 for the iPod. Please be specific about your issues if you find your statements to be worthwhile and worth reading. Sounds to me like your issue is more with the player(s) that you use, rather than with mp3tag.
When I convert my FLACs to MP3 for my iPod I convert ARTISTSORT, ALBUMSORT, and ALBUMARTISTSORT to TSOP, TSOA, and TSO2, respectively.
Which players recognizes ARTISTSORTORDER in FLAC files? I know ARTISTSORT is widely recognized.
My issue is... I have about 22 action groups defined right now, but they all operate on FLAC files (they also work well on Ogg), with free-form field names. They do not work correctly on MP3 files, because if I want a field named COMPILATION in the FLAC, it does me no good in the MP3 as a TXXX field.
I hope to see a solution from Mp3tag soon in the form of flexible mapping of internal names to fields in various tagging systems. I want to be able to define what 'ARTISTSORT' is going to translate to in a particular system.
For the above problem, I'm trying a new workaround for MP3s. I'm creating an action group that I can check off and run after all the FLAC action groups have executed. The FLAC action groups will end up creating a load of TXXX fields in the MP3s, since few of the field names are recognized by Mp3tag, then the MP3 action group will copy those fields into the correct ID3v2 fields by using the necessary Mp3tag field names. Then is deletes all the TXXX fields. Should mostly work, but it's convoluted, and it also can't be used for the fields in the Tag Panel. If I use the Tag Panel, I have to execute the MP3 action group afterward.
I'm just thankful that I don't also have to make it all work with WMA and AAC files. 
I'm sorry, that was an overly harsh criticism, and uncalled for. I think Florian knows how much I appreciate all of his hard work on Mp3tag. Quite the contrary, Mp3tag has progressed enormously in the years that I've used it.
I just wish it were easier to use with more than one tagging system. 
It does not matter what the actual tag name is for MP3, so long as the TSOP frame is written to. If TSOP is written, the player used will show the entry and will show its own tag name. When converting from FLAC, have your FLAC files tagged as your converting software prefers (Vorbis Comments are great for this). You can add tag names freely in mp3tag. The problem is not mp3tag, but each player using different tag names and ID3 versions 2.3 and 2.4 having different standards. iTunes also muddied up the waters by including some v2.4 standard sorting tag frames which in v2.3 makes them non-standard and sometimes not viewable in certain players. It is a mess, but not because of mp3tag.
We're not communicating. You haven't understood what I'm saying. It has nothing to do with ID3v2 tags or their interpretation by applications. I thought I explained it pretty clearly in Post #9.
Sorry to have taken this thread off topic.