Campaign Budget Optimization (CBO), now called Advantage campaign budget, sets one budget at the campaign level and lets Meta distribute it across ad sets in real time toward the best performers. It is how you scale sideways without hand-splitting spend.
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.