Is a 50Amp 240 Volt Circuit Breaker (double breaker) rated for 25 amps on each side or 50 amps on each side?

If you trace a resistive (R) circuit powered by the 240 V source, you fill find it starts at the source, goes through CBa then R, then CBb and then returns to the other side of the source.

CBa should equal CBb if the circuit were rated for 50A.