A single customer might see a creator's ad, close the app, search the brand later, and buy from a different link days after. Attribution is the set of rules a platform uses to decide, out of every touchpoint in that path, which one gets credit for the resulting sale. Different rules produce different answers for the exact same purchase.
Meta's ad attribution counts a sale if someone interacted with the ad within its attribution window, typically a click within a set number of days or a view within a shorter one. Shopify's last-click model instead credits whichever link the customer clicked immediately before checkout, no matter what they saw earlier. A creator's content can influence a sale that a last-click model never credits to them.
On Socialync's job board, an ads deal states upfront whether commission is measured by Meta attribution or Shopify last-click attribution, since the choice materially changes what counts as a result a creator gets paid for.