I'd like to add my thanks for the fabulous work done in supporting Matroska files so far.
I also think it'd be great if you could somehow implement user-defined mapping functionality, including the target type levels, for Matroska tags.
A mapping I'd particularly like to add, which I think would make sense to have as a default, if it could be done more quickly that way, would be from Mp3tag's TVSEASON field to Matroska's PART_NUMBER tag at target type level 60 (T=60).
If I create this T=60 PART_NUMBER Matroska tag longhand (by writing an XML file and muxing in via via MKVToolNix rather than using Mp3tag), then MS Windows will display the held number value for me as Season number (Windows property name = System.Media.SeasonNumber).
If I use Mp3tag's TVSEASON field when tagging an MP4 video file, that'll also display the same way in Windows.
So, it'd make sense if the TVSEASON field worked in a consistent way, to facilitate Windows showing a season number for Matroska video too.
Similarly for TVEPISODE as well.
If I use Mp3tag's TVEPISODE field when tagging an MP4 video file, that'll display as Episode number in Windows (System.Media.EpisodeNumber).
Windows would also display an Episode number for MKV video if Mp3tag's TVEPISODE field were mapped to Matroska's PART_NUMBER tag at target type level 50 (T=50).
Further notes:
There is a workaround to get a Matroska PART_NUMBER tag created at target type level 50 (T=50) for the purposes of an episode number, which is to use Mp3tag's DISCNUMBER field instead of TVEPISODE.
An Mp3tag DISCNUMBER entered numeric value will display in Windows as Episode number.
I think mapping TVEPISODE to T=50 PART_NUMBER would be better for MKV video though, making it more consistent with the way MP4 video tagging works for example.
I don't think there's any workaround to create a PART_NUMBER tag at target type level 60 (T=60) in Mp3tag (thus displaying in Windows as Season number).
TVSEASON mapping to T=60 PART_NUMBER would therefore the priority request from that point of view.
However, bear in mind also, while I don't think there's a currently a way to create a Matroska T=60 PART_NUMBER tag using Mp3tag; it will display a pre-existing one (a tag created by writing an XML file and muxing via MKVToolNix, say).
Mp3tag displays this T=60 PART_NUMBER value in its DISCNUMBER field, along with any T=50 PART_NUMBER, showing the two separated by a double backslash.
For an MKV video file with both a season number tag and an episode number tag, Mp3tag would show, for example, T=60 PART_NUMBER holding the value 61, and T=50 PART_NUMBER holding the value 58, as DISCNUMBER = 61\\58
If you try and edit this displayed DISCNUMBER value, say, from 61\\58 to 61\\59, then that has the effect of disappearing the underlying MKV file's original T=60 level PART_NUMBER, and creating its held value at level T=50 instead. Thus losing the season number and ending up with two episode level T=50 PART_NUMBER tags, one holding a value of 61, and the other holding an updated value of 59.
I'd suggest the solution to that issue is to have Mp3tag's DISCNUMBER field functionality fixed, to ignore and leave well alone any T=60 PART_NUMBER tags.
Version and OS info:
At time of writing I'm using the most current Mp3tag version, v3.23
OS Name: Microsoft Windows 10 Pro (version: 10.0.19045 Build 19045).
TL;DR Summary
For MKV files:
Mp3tag's TVSEASON field should be mapped to Matroska's PART_NUMBER T=60 tag.
Mp3tag's TVEPISODE field should be mapped to Matroska's PART_NUMBER T=50 tag.
+
Mp3tag's DISCNUMBER field shouldn't display/edit any PART_NUMBER T=60 tags.