EU-Ops.
From memory:
Class A - defined performance for continued flight after EF at V1. A is for Airliner.
Class B - no specific performance required at any stage of flight following an engine failure. B is for Bugsmasher.
Class C - large or heavy or turbojet aircraft that really should be in Class A, but lack the OEI performance. C is for Classic aeroplane, think DC3.
I don't recall the precise weight / seat numbers / powerplant requirements that distinguish the categories.