A segment is a saved audience: a set of rules over loyalty state that selects which pass holders a message should reach. Rules combine conditions such as points balance, stamps collected, tier, last visit, wallet type, birthday month, tags and custom attributes, "Gold members who have not visited in 60 days", "iPhone holders with an unredeemed bonus check".
Segments are live, not snapshots. Membership is evaluated when the segment is used, so a member who crosses a threshold flows in, and one who no longer qualifies flows out. A saved segment therefore behaves like a standing question about the program, and its size over time is a metric in itself: watching "lapsing members" grow or shrink says more than most dashboards.
Their first job is targeting campaigns, where a segment plus a message plus a schedule is the whole recipe, and estimated reach previews the audience before sending. The same rule vocabulary conditions automations, so a trigger can apply only to a slice of the program.
Segments are also the discipline that keeps lock screen messaging welcome. The wallet channel is precise enough to say something relevant to two hundred people instead of something generic to twenty thousand, and programs that use it that way keep their place next to the payment cards. In Stell, segments live in Engage, the messaging module.