How a segment works

A segment is a saved rule that narrows one list to part of its contacts. It stores conditions, not people: the rule is re-resolved every time it is used, so a segment written in March keeps working in December on whoever matches then. A segment lives inside a single list, and its name is unique within that list.

Building the rule

A condition is a field, an operator and a value:
FieldOperators
taghas · does not have
opt-in date · creation datebefore · after · is set · is empty
first name · last name · notescontains · is set · is empty
Conditions are collected into groups. Inside a group you choose AND (all conditions must hold) or OR (at least one). Between groups you choose again. That is enough to express things like "carries customer AND signed up before June" or "carries beta" — without the rule becoming unreadable. The builder shows the equivalent rule in plain words as you go, and a live count of how many contacts it reaches.
An AND group can quietly reach nobody. Two conditions that cannot both be true — a tag the people who signed up before that date never carried, for instance — give a count of zero. Zero is a valid rule, not an error, and a campaign will happily send it to nobody.

Who it can reach

A segment only ever selects among the subscribers of its list. Unsubscribed contacts, bounced addresses and anything on the suppression list are out of reach by construction — no rule brings them back. Which is also why the count in the builder is an estimate as of today. The real audience is computed at send time: people unsubscribe, get tagged, or come in through a form in the meantime.

What refuses

Deleting a segment that a campaign targets is refused, and the message says how many campaigns stand in the way. Deleting one that nothing uses removes the rule only — contacts and tags are untouched, since a segment never owned them. Deleting a tag used by a segment is refused for the same reason: the rule would silently change meaning.

What it implies elsewhere

A segment narrows, never widens: its audience is always a subset of its list. Targeting both a tag and a segment on the same campaign stacks the two filters. And because a segment belongs to one list, it cannot be reused on another — the same rule on a second list is a second segment. Agents build and reuse segments through the API with the same vocabulary.