1. Class A: Non-combustible building materials: Materials that hardly burn.
2. Grade B1: Flame retardant building materials: Flame retardant materials have good flame retardant effects. It is difficult to catch fire when exposed to fire in the air or under high temperature, and it is not easy to spread quickly, and the combustion stops immediately after the fire source is removed.
3. Grade B2: Combustible building materials: Combustible materials have a certain flame retardant effect. In the air, when exposed to fire or under high temperature, it will immediately ignite and burn, which may lead to the spread of fire, such as wooden columns, wooden trusses, wooden beams, wooden stairs, etc.
4. Grade B3: Flammable building materials: no flame retardant effect, extremely flammable, and dangerous fire.