Which extinguisher classes do NOT have number and letter ratings?

The classes of fire extinguishers that do not have number and letter ratings are specifically C, D, and K. Class C extinguishers are designed for electrical fires and are labeled with a "C," but they do not use a numerical rating because their effectiveness cannot be quantified like Class A extinguishers, which deal with ordinary combustibles. Class D extinguishers are used for combustible metals and have specific applications that also do not lend themselves to a standardized numerical rating. Similarly, Class K extinguishers are meant for cooking oils and fats, particularly in commercial kitchens, and they carry a "K" designation without a numerical value, as each type is used for particular scenarios rather than graded by effectiveness like Classes A and B.

Other classes, like A and B, receive both letter and number ratings because their effectiveness in extinguishing fires can be quantitatively compared based on the volume of material they can extinguish.