All optimisation rules are configured at the Adset level to efficiently manage performance and not conflict with Facebook’s native optimisation algorithms.
Facebook automatically measures the best performing advert in an adset, see When should I add more than one Advert to an Adset. StitcherAds layers additional optimisation to help you identify the best performing audiences for a particular product or sale. When you select a rule for a particular campaign, that rule is automatically applied to all adsets in that campaign.
Sometimes an adset might break your optimisation rule while all other adsets in that campaign are performing well. In this case, you would want the relevant rule (pause adset) to only apply to the adset that is not performing.