I'm actually curious to see what that USR file looks like inside.
Two of my Yammies use USR files to save and load "user backups" of the User Songs, Registrations, and User Styles. And all three of my Yammies use BUP files to save and load backups. The BUP and USR files are very similar in structure, but the BUP files also contain the panel settings which are preserved when you turn the power off. The advantage of USR files on my Yammies is that they can be transferred to or from a USB thumb drive without having to be connected to a computer, whereas BUP files must be transferred to or from a computer using special software (either the Yamaha Musicsoft Downloader for Windows, or the Yamaha MusicSoft Manager for iOS devices).
Anyway, I'm curious about how similar the MM6's USR files are to those of the PSR-E433 and PSR-E443. On page 69 of the MM6/MM8 Owner's Manual it says that a USR file contains the User Songs, Pattern File, and Performance Memory Data, which sound like the equivalent of User Styles (Pattern File) and Registrations (Performance Memory Data). So if you were going to be playing your MM6 in a gig, you could save some User Songs, a Pattern File, and Performance Memory Data to a USR file, then save some different User Songs, Pattern File, and Performance Memory Data to a different USR file, etc., such that you have a collection of USR files, presumably with the intention of playing five musical numbers per USR file-- or more, if some of the numbers use the same User Songs as each other (see below). Then, when you finish playing one set of numbers using one USR file, you can load a different USR file for the next set of numbers. But it takes a little while for a USR file to load-- at least, it does on my Yammies-- so you'd have to make sure the singer goes into a bit of patter while you're loading the next USR file.
On page 29 it says you can mute song tracks. Since there are eight melody tracks, you could split them up between different numbers-- e.g., for one number you could play back User Song 1 with all tracks muted except for Track 1; for another number you could mute all tracks except for Track 2; etc. That way you could use a single User Song for up to eight numbers (one track per number), or fewer than eight numbers (multiple tracks per number). However, keep in mind that the Tempo gets saved in the User Song, so each track of a given User Song will play back at the same Tempo.