Estimated reach is the pre-send truth: before a campaign goes out, it states how many people will actually receive it. The number counts pass holders matching the audience, contacts with a pass currently installed in a wallet, because a lock screen message can only land where a pass exists to carry it.
That definition makes reach an honest metric in a way database counts are not. A segment might match ten thousand members in the CRM while only six thousand of them hold a pass; estimated reach reports six thousand, because that is who the send can touch. The gap between the two numbers is itself a finding: it measures how much of the program has adopted the wallet channel, and it points at enrollment as the lever when reach feels small.
Practically, reach is the sanity check in the send flow. A campaign meant for a narrow slice showing a five-figure reach has a broken rule; an offer meant for the whole base showing a three-figure reach has an adoption problem or an over-tight segment. Either way, the number surfaces the mistake while it is still free to fix.
Reach is an estimate because the population moves, passes are added and deleted continuously, but it is computed over live pass-holder state at the moment of asking. In Stell, estimated reach appears in Engage, the messaging module, on both campaigns and segments.