What is Stop on reply?
The rule that any inbound reply from a prospect immediately cancels every remaining step of their sequence, before a human reads it.
Sending step 3 to someone who answered step 2 is the most damaging thing a sequence can do — it reads as automation, invites a spam report, and can lose a warm lead. Stop-on-reply must be automatic, immediate and triggered by any reply, including "not interested" and out-of-office (with out-of-office ideally holding rather than ending). Most tools also offer stop-on-click or stop-on-open, which are weaker signals.