C is for comfort. It's their av system. Not sure why it's particularly specified. Any decent saw, and most crappy ones have av now. M is for Mtronic. Their electronic control system. A lot of people don't like for reasons I don't quite understand. My best guess is it's due to possible early problems with the design. A real potential problem is if you have trouble with it, going to a Stihl dealer is mandatory. You can't simply work on the carb to get it right. It has to be hooked up to a computer running Stihl's proprietary software.
I like Mtronic a lot. It just works. Since I didn't get a manual with my first Stihl, and the dealer didn't tell me anything, I didn't know it even had a cold start procedure. I just pulled the cord, and it would fire after ~7 pulls. I now know it has a choke/fast idle setting that shaves a couple pulls off that for first start. It adjusts for cutting conditions, elevation, temperature, and cleanliness of the air filter. No monkeying around with carb settings. It Just Works®