The numbers and letters on a fire extinguisher, such as 5BC, indicate the kinds of fires it should be used on, as well as the extinguisher's fire-suppression capability.
The National Fire Protection Association identifies five categories of fires: A, B, C, D and K. The letters on a fire extinguisher correspond to these categories.
A 5BC extinguisher puts out B- and C-class fires. B-class fires involve flammable liquids such as gasoline or grease. C-class fires involve electrical equipment. The "5" indicates, roughly, how many square feet of fire the extinguisher can put out.
Water will not put out B and C fires, so BC extinguishers are usually filled with a dry chemical--sodium bicarbonate or potassium bicarbonate--that smothers the fire. Some BC extinguishers use carbon dioxide.
Dry-chemical extinguishers leave a corrosive residue; overspray must be cleaned off immediately to prevent damage. A carbon dioxide extinguisher is a better choice around sensitive equipment.
The other fire classes are: A, ordinary combustibles such as wood, paper and cloth; D, combustible metals, usually found only in industrial applications; and K, cooking oil and grease, mostly in industrial kitchens (a B-rated extinguisher is fine for residential kitchens).