Foam fire extinguishers can be used on Class A and B fires. They are most suited to extinguishing liquid fires such as petrol or diesel and are more versatile than water jet extinguishers because they can also be used on solids such as wood and paper.
It is recommended to use a foam fire extinguisher for class A and class B fires. Fires that are categorized as class A involve flammable solids such as rubber, wood, paper and textile materials, where as class B fires involve flammable liquids such as kerosene or petrol.
You would typically see these fire extinguishers in racing applications where alcohol fuel is utilized.