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A segment is an audience defined by rules, not by hand. Instead of adding people to it, you describe who belongs — “clicked in the last 7 days and hasn’t converted” — and membership recalculates itself as contacts act. It’s the dynamic counterpart to a list (a static set you add people to). A campaign can target either.

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Build a segment

1

Add conditions

Each condition is a field, an operator, and a value — e.g. Total clicks · at least · 1, or Last replied · within last · 14 days.
2

Combine with all / any

Conditions sit in a group that matches all of them (AND) or any of them (OR). Add more groups to express subsets like “(A and B) or C.”
3

Preview the size

The builder shows how many contacts currently match as you edit, so you never build a campaign against an audience of zero (or of everyone). You can also preview over the API before saving.
4

Save and use it

Save the segment, then pick it as a campaign’s audience, or snapshot it to a list to freeze the current members as a static set.

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What you can segment on

Engagement Recency (within last / older than N days) Conversions Attributes

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Segments vs lists

Orphans. A segment can include or exclude contacts who aren’t on any list (the accept_orphans setting), so an audience built purely from behavior isn’t accidentally limited to list members.

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Snapshot to a list

Segment membership is live, which is usually what you want — but sometimes you need a frozen set (to send to exactly these people, or to hand off elsewhere). Snapshot to list copies the current members into a new static list, leaving the segment untouched.

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Over the API

Build and size segments programmatically — Create a segment (POST /v1/segments), Preview a segment count (POST /v1/segments/preview), and List segment members — or in natural language through the MCP server.