Final answer:
A rating number is required for Class A and Class B fire extinguishers.
Step-by-step explanation:
In regards to fire extinguishers, a rating number preceding the classification letter is required for Class A and Class B extinguishers.
Class A extinguishers are used for ordinary combustible materials like wood or paper, while Class B extinguishers are used for flammable liquids or gases.
For example, a common rating for a Class A extinguisher is 2A, which indicates its effectiveness against common combustibles.