RC (Reserve Capacity) - the number of minutes that the battery can deliver 25 amps while keeping its voltage above 10.5 volts.
Typically a deep discharge battery will have 2 or 3 times the RC of a car battery, but will deliver one half or three quarters the CCAs. In addition, a deep cycle battery can withstand several hundred total discharge/recharge cycles, while a car battery is not designed to be totally discharged.