Creative testing
Running several versions of an ad against the same audience to find out which one that audience actually responds to.
In paid social the creative does most of the targeting. The platform will go and find the people who respond to a piece of work, so what the work says decides who sees it more than the audience settings do.
A test is only a test if one thing changes. Three ads with different images, different headlines and different offers will produce a winner and no information about why it won, which means nothing to apply to the next one.
Budget should follow the result rather than the plan. Within a few weeks almost all of the spend on a healthy account sits behind the one or two ideas that earned it, and the others were the price of finding out.