The numbers on fire extinguishers refer to the extinguisher's UL (Underwriters Laboratories) rating, which is a measure of the extinguisher's effectiveness in putting out fires of a particular type and size.
The number indicates the extinguisher's effectiveness on a particular class of fire. The higher the number, the more effective the extinguisher is at putting out that type of fire. For example, a fire extinguisher with a UL rating of 2A would be more effective on a Class A fire than one with a rating of 1A.
