Thank you for this useful information an links.
With the help of this Open Source tageditor I was able to fill the "default" mkv tags in the Levels 30, 50 (and 60 collection level) and this is how they look in Mp3tag:
So you are right (as expected):
The L30 Artist would be filled into Mp3tag ARTIST
The L30 Title would be filled into Mp3tag TITLE
The L50 Artist will be filled into Mp3tag ALBUMARTIST