A fire-endurance rating for doors, shutters, etc., established by the Underwriters’ Laboratories, Inc., or other recognized and approved laboratory:
class A: 3 hr; for doorways or other openings through a wall separating buildings or dividing a single building into fire areas;
class B: 1 or 1½ hr; for doorways or other openings in enclosures of vertical transportation through buildings (stairs, elevators, etc.);
class C: 3/4 hr; doors in corridor and room partitions;
class D: 1½ hr; doors and shutters in exterior walls which are subject to severe fire exposure from outside the building;
classes E and F: 3/4 hr; doors, shutters, or windows in exterior walls which are subject to moderate or light fire exposure respectively from outside the building.