Ad Set Budget gives the advertiser direct control over how much each ad set spends, rather than letting Meta's delivery system redistribute a shared pool. This is the standard choice when testing multiple audiences or creative concepts side by side, since each variant is guaranteed its allotted spend and can be judged on a level playing field.
The tradeoff is more manual oversight: a media buyer has to actively monitor and reallocate budget between ad sets as winners and losers emerge, which is more labor-intensive at scale than CBO but gives tighter control, a common tradeoff agencies weigh when running dozens of concurrent tests.

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