A campaign sets out what is being promoted, what source material is available, what the guidelines are, and what rate applies to views. Clippers discover it, join it, and submit clips against it.
The brief is the highest-leverage part. Clippers cannot read a marketer's mind, and a campaign that specifies its non-negotiables clearly — tone, claims to avoid, assets to use — gets usable clips more often than one that does not.
Campaigns are bounded by a budget. Earnings are performance-based and depend on the views a clip receives, so there is no guaranteed amount for any individual clipper.