The 'C' rating is a rating of safe discharge rate. The stock batteries on an X8C are 2000mah. If they were rated 10C, it would mean that the battery is designed to be safely discharged at 10x its capacity, or 10 x 2200mah: in other words 22000ma, or 22 Amps maximum continuous discharge. A 20C battery can safely be discharged at 44 Amps, etc. Higher is (at least theoretically) better both from a safety and performance standpoint. I think most people would agree that batteries used for quads should have at least a 25C rating.
