Why does a circuit breaker not show which terminal is IN and OUT?
Because it's irrelevant. Circuit breakers have no polarity. That is, there IS no "in" and "out". You can hook it up either way.
For AC breakers, yes.
DC circuit breakers may have a polarity. Those that do oppose quenching and direct the arc into the switching mechanism rather than aid quenching, and point it into the arc chute when they are wired backwards.
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