Segments
Named groups of contacts that campaigns send to.
A segment is a named audience — a group of contacts a campaign can target. Each campaign sends to exactly one segment, so segments are how you decide who receives a given send.
The list shows each segment's subscribed-member count alongside its total. Only subscribed members are mailable, so this is the number that matters: a segment with zero subscribed contacts is flagged, because a campaign to it can't actually send.
Empty segments block sends
A campaign whose segment has no subscribed members is stopped before it sends — there's no one to mail. Watch for the "0 subscribed" / "Empty" badge in the list and fix the audience before scheduling.
Archiving & deletion
Deleting a segment archives it. An archived segment disappears from campaign pickers and can't be targeted by new campaigns, but the historical record of campaigns that already used it stays intact. You can restore an archived segment, and the system permanently removes it ~30 days later once nothing active still references it.
Can't archive while actively referenced
If a non-terminal campaign (draft, scheduled, or sending) still targets a segment, archiving is blocked with a 409 — cancel or finish that campaign first. A segment used only by sent/finished campaigns archives cleanly. This is the defined, tested behavior for deleting a segment that a campaign references.