I'm not sure about the M1A or B, but the M1R and L is for right and left. For the M1R, you pick up the horizontal strand with your left needle moving from back to front, and knit into the front of the stitch. With the M1R, you pick up the strand with your right needle, moving from back to front, and knit into the back of the stitch. I've used them to knit raglan increases on sweaters, and I think they lie a little neater, at least for me. Hope that helps!