i don't know what it means but here's an idea: compare your friend against their numbers ans see if you can actually work out what it means. If you put them in a line by number and they get fatter as you go along, or shorter or uglier then you will know lol I know its not very nice to do that to your firneds, but it's a way to find out. :)
G6. Like the plane. This friend must be supa-flyyyyy :)
Haha, all joking aside, that is one very complicated rating system. Why don't you ask one of your guy friends about it? The whole numbers and letters thing is very arbitrary, no matter which rating scale you look at.
Personally I wouldn't bother sweating about what M3 means, just rock being an M3 :)