"ABO," or ad setbudget optimization, refers to setting individual budgets for your ad sets (audiences). Unlike a CBO campaign , you know in advance how much budget an ad set will spend, and it is not dependent on which ad set the Facebook algorithm identifies as having the best opportunities. According to Facebook, an "ABO campaign" can be useful if you have combined optimization goals and bidding strategies, or if there are significant differences in the audience sizes of the various ad sets. In a CBO campaign, larger audiences are almost always prioritized.