Both delay general alarm so trained staff can investigate first — but only one of them has a code-mandated stopwatch, and a fail-safe that fires if nobody answers in time.
The companion Concept Explainer on alarm, supervisory & trouble signals covers what a signal means once the panel reports it as a general alarm. This one covers a step that can happen before that: a deliberate, engineered pause — a signal that goes first to a specific, staffed location instead of triggering building-wide notification immediately. That broader idea is called presignal operation. Positive Alarm Sequence (PAS)is often described as if it were just presignal by another name. It isn't.
PAS is one specific, tightly bounded, code-approved way to implement the presignal concept: NFPA 72 §23.8.1 caps the acknowledgment window, caps the total investigation time, restricts it to automatic-detector signals only, and — critically — forces general alarm automatically if the sequence isn't properly closed out in time. A generic presignal scheme isn't required to have any of that. That gap is exactly why an unconstrained presignal delay was historically linked to preventable delayed-evacuation deaths, and why the code doesn't simply permit "presignal" — it permits PAS.