Originally Posted by rickair7777

Class 3: Recreational (non-compensated) flying only. This means part 91 private-pilot stuff.
Class 2: Legally required for almost any flying which requires a commercial certificate. Exception is that you do not need a medical to flight instruct, as long as the student is rated to act as PIC for the flight.
Class 1: Legally required for any flying which requires an ATP (airline Captain, and probably also airline FO in three years). However many non-121 operations require a first class as a matter of policy, and any airline will require one to get hired initially.