Messaging rules
Segment
A defined audience — either an explicit list of contacts, or a saved filter that recalculates its own membership.
Messaging rules
A static segment is a list you chose. A dynamic segment is a rule — "tagged trial, no order in 60 days" — whose membership is recalculated when you use it.
The difference matters at send time. A static list exported on Monday is wrong by Friday; a dynamic segment is right whenever you send, which is the whole argument for using them on recurring campaigns.
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