When fire extinguishers are rated the letters and the numbers have specific meanings.
Class A is for ordinary combustibles, and the number is the equivalency of 1 1/4 gallons of water extinguishing capability. A 1A is equal to 1 1/4 gal, a 2A is equal to 2 1/2 gal, a 10A is equal to 12 1/2 gal, and so on.
Class B is used for flammable liquids. The number is the equivalant of square ft of heptane that an untrained user would be expected to extinguish. A 1B would extinguish 1 sq. ft, a 2B would extinguish 2 sq. ft., a 10B would extinguish 10 sq. ft., and so on.
Class C is an extinguishing agent that is safe to use on fires involving live electrical equipment. The electrical equipment itself isn't the fuel, but it presents an electrocution hazard. The fuel would be either a Class A or a Class B, therefore you need an extinguisher with a multiple rating.
Then there is Class D, which is combustible metals, such magnesium.
