I believe the M1 sound is coupled to the Melody Intelligence. For certain Melody Intelligence settings, you will get a harmony voice added to the Right1 part that is a different voice (instument) than the R1. Some Melody Intelligence parts merely add melodic harmonies of the same R1 voice. Others let you select the M1 voice (such as sax) that layers the harmony with the chosen R1 voice (say, piano).
Then again, I could look in the manual to be sure. But, it's late & I'm tired.
Tom