This is a suggestion to allow the command 'Tools >> Tagging Tools >> Renumber Tracks' to optionally write the new track numbers into a different tag that the user chooses (especially a custom tag).
Basically, one might want to do this do create a personalized track order without changing the canonical track numbers that come from the album. This is a purpose similiar to what playlists provide, but using tags to order tracks instead of a playlist is more flexible (the tags work in every playlist you put the album into) and more robust (the tags travel with the file wherever it goes, while a playlist will break when files and folders move). I also posted about using this capability for a more specialized purpose here:
http://getmusicbee.com/forum/index.php?topic=8433.0