List or segment as an automation trigger?

Both can start an automation, and they behave differently in a way that decides whether people can enter twice.

Added to an audience

A one-off act: someone is put in a list, and they enter the flow.

Use it when you control entry, and when the flow should run once per person. It is the predictable choice, and the safer default.

Entered a segment

A state change: someone starts matching a rule.

Use it when entry should follow behaviour — became a VIP, lapsed, browsed without buying. Nobody has to remember to add them.

The catch: if a contact stops matching and matches again later, they can enter again. For a win-back flow that is correct. For a welcome series it is embarrassing.

The rule of thumb

• Should this run exactly once per person? → list.
• Should this run whenever the condition becomes true? → segment.

Timing differs too

A segment trigger fires when the segment is next evaluated, not at the moment the underlying event happened — so there is a lag. For genuinely immediate reactions, a custom event is the right trigger. See Automation triggers and When do segments update?.

Entry is not sendability

Whichever you choose, contacts who cannot be emailed still enter the flow and still progress — they are simply not sent to. So a busy trigger does not guarantee mail going out, and a flow with healthy enrolment and no sends is usually a consent problem rather than a broken automation.

Next: open your automation's Trigger node and check whether it should really be able to fire twice for the same person.