First-click attribution, also called first-touch attribution, assigns the whole value of a conversion to the first recorded interaction. If someone found you through a TikTok ad, came back through a Google search and finally bought from an email, TikTok gets 100%.
It is the best model for judging prospecting: the campaigns whose job is to introduce new people to your brand. Under last-click attribution these campaigns often look unprofitable, because someone else closes the sale.