A frequency cap is a per-contact ceiling on messaging: no matter how many automations a member triggers, the cap limits how many messages they receive in a given window. It is the safety rail that lets a program run many triggers without any one member experiencing them as a pile-up.
The failure mode it prevents is easy to reach. A member earns points, crosses a tier threshold and hits their pass anniversary in the same week; three well-intentioned automations each have something to say, and without a cap the lock screen says all of it. Each message was individually reasonable. The sum is spam, and on a wallet pass the cost of spam is concrete: deleting a pass is one swipe, and a deleted pass ends the channel for that member entirely.
A cap encodes a judgment about the channel's intimacy. The lock screen sits closer to a person than an inbox, so wallet messaging tolerates far less volume than email; a cap makes that tolerance an enforced property of the system rather than a hope about restraint across everyone who can create an automation.
Caps govern automations, whose volume is emergent from member behavior. Campaigns are deliberate, scheduled sends, so their volume is a decision made by a person. In Stell, the frequency cap is part of Engage, the messaging module.