Lists vs segments: which to use

A list is a named collection a contact belongs to, like "Newsletter" or "Customers". A contact can be on several. Subscription status is per list, so unsubscribing from the newsletter does not remove them from transactional lists.

A segment is a saved filter across all contacts: "opened in the last 30 days", "in Wisconsin", "custom.plan = pro". Segments are live; the count changes as data changes.

Use lists for intent. Use segments for targeting.