These classes describe the distance the SRL can unwind before stopping and the amount of force allowed on the worker. The classes are defined in the ANSI Z359.14 fall protection standard:
Class A SRL devices must limit average fall arrest forces to a maximum of 1350 lbs. and limit potential lifeline deployment to 24 inches or lessClass B SRL devices must limit average fall arrest forces to a maximum of 900 lbs. and limit potential lifeline deployment to 54 inches or less.   Both Class A and Class B SRL devices must limit the maximum arresting force to 1800 lbs. or less