What is an optimisation event on Meta?

As a column, Meta says Optimisation events is calculated as the number of optimised events that occurred during your chosen conversion window. For outcomes where time may pass between someone seeing the ad and the event, Meta uses an attribution window that corresponds to the conversion window chosen during ad set creation: with a conversion window of 1-day click and 7-day view, the column counts outcomes on 1-day click and 7-day view.

Meta says it can be useful to evaluate the number of optimisation events you've received since your last significant edit. It recommends around 50 optimisation events, but says some ad sets stabilise earlier. Its learning limited test uses the same count: an ad set becomes learning limited when it is unlikely to receive about 50 optimisation events in the week after your last significant edit. Any change to the optimisation event is on Meta's list of significant edits.

Meta says performance is optimised to minimise each ad's cost per optimisation event. Our reading, not Meta's: the optimisation event is the number the delivery system is trying to make cheap, so if an ad set optimises for link clicks under a sales objective, a low cost per optimisation event is a statement about clicks and says nothing about sales.