Campaign budget optimization is a way of optimizing the distribution of a campaign budget across your campaign's ad sets. This means Facebook automatically and continuously finds the best available opportunities for results across your ad sets and distributes your campaign budget in real time to get those results. Starting in September 2019, CBO will be mandatory for all campaigns.
You can select to use CBO in AdRules in the Campaign Step of AdBuilder. Check on the Use Campaign Budget Optimization box and select your daily or lifetime budget.
You can set Ad Set spend limits in the Budget Step of AdBuilder.
You can later change Ad Set spend limits directly from Advanced Reporting. Select the Ad Set you'd like to modify, then click on Actions, Edit Ad Set, & Budget.