Its very very simple

Class A: Device conducts for full signal cycle
Class B: Device shut off for some of cycle, but conducts for more or equal to half the cycle
Class C: Device conducts for less than half the cycle.

For push pull, As long as the signal level is such that neither valve goes into shutoff then its class A, once one valve is off then its B. Its possible to bias the valve such that neither valve shuts off with full input signal, Class A. Bias so that both are on for some of the time but as the signal increases one shuts off, then its AB, Bias it so at idle both are shutoff, and they are never on together, but never both off when there is a signal, then its Class B. Bias it so there is a dead zone where both are off its Class C.

(the above is equally true for transistors as valves).

If no grid current flows at any time, its A1, B1, or C1 (the 1 is normally omitted). If grid current flows for some or all the time, its A2, B2 or C2.

Its all In RDH (but if you know what that is, you already know the above).