When in foobar2000 0.9.x I save DISCNUMBER tag (in WavPack file) it really saves tag as DISC. Then I want replace it back in Mp3tag. When I open file's tags by I can see "DISC = 1" in the list of tags. After double click on this line Mp3tag itself replaces DISC on DISCNUMBER in tag title field (but I can make it myself with any tag). After saving changes in the list of tags line "DISC = 1" replaced by "DISCNUMBER = 1" but really DISC was not deleted. You can check this Applying changes to file and open tags by again.