How it works
Instead of a fixed budget on each ad set, you set a single campaign budget and Meta shifts it toward whichever ad sets are converting most efficiently at that moment. This is the mechanism behind horizontal scaling: add new ad sets and let the campaign fund the winners.
Where it helps and hurts
CBO shines when you are testing several audiences and want the algorithm, not a spreadsheet, to find the winner. It can frustrate when you need guaranteed spend on a specific audience, since Meta may starve an ad set it judges weaker. It pairs naturally with a lookalike of your converters as one of the tested ad sets.
Common questions
Is CBO the same as Advantage+ campaign budget?
Yes. Meta renamed Campaign Budget Optimization to Advantage+ campaign budget; the mechanic, one budget shared across ad sets by the algorithm, is the same.
When should you not use CBO?
When you need to guarantee spend on a particular audience or test ad sets at equal budget, ad-set budgets give you more control. CBO optimizes for the campaign, not for even distribution.