What does the letter "b" on a b-1 fire extinguisher indicate?

The B indicates the fire extinguisher will put out Flammable liquid fires or burning fluids (such as grease or oil used for cooking). Class A extinguishers are good for paper, wood, textiles and plastics. Knowing what the type of fire extinguisher is for is important so that it can be suited to the type of fire as one needs to act fast in case of a fire.