Groups added under Exclude are resolved to phone numbers the same way as the To groups, and any number that appears in both is removed from the send. For example, send to "All users" and exclude "All Users With Active Subscription" to reach only non-subscribers. The Review & Confirm window lists the Excluded groups with their counts, and the Recipients number already reflects the exclusion. Recurring campaigns store their excluded groups too and re-apply them on every run.