Final answer:
The numerical rating on Class B extinguishers refers to the fire-extinguishing capability of the extinguisher against Class B fires, which involve flammable liquids or gases.
Step-by-step explanation:
The numerical rating on Class B extinguishers refers to the fire-extinguishing capability of the extinguisher against Class B fires, which involve flammable liquids or gases. The rating is based on the size of the fire that the extinguisher can handle. For example, a 1-B rated extinguisher can handle a fire involving approximately a square foot of flammable liquid, while a 640-B rated extinguisher can handle a fire involving approximately 640 square feet of flammable liquid.