Describe a scenario where you would use a segmentation rule instead of an automation rule.

Segmentation rules run once and stop, so they suit one-off historical clean-up. A concrete case: we introduce a new region field, and 40,000 existing prospects have a country value but no region. A segmentation rule can run once over the database and populate the region, then never run again.

An automation rule would be the wrong tool there — it would keep evaluating the whole database indefinitely, and each org has a limited number of active automation rules, so spending one on a job that is finished wastes a scarce resource.

Conversely, when the logic needs to apply to prospects who arrive next month too — assigning MQLs, for instance — the automation rule is correct precisely because it keeps running.