Ad Set Budget Optimization is the original way Meta budgets worked: every ad set gets its own daily or lifetime budget that you set manually. ABO is now the opt-in instead of the default — CBO is the default — but it's still the right choice for several specific scenarios.
When ABO is the right call
- Testing-phase: each ad set gets fixed equal spend so you can compare audiences cleanly
- Mixed-funnel campaigns where prospecting + retargeting share a campaign and you don't want CBO to defund prospecting
- Per-audience budget caps imposed by a client or by compliance
- Audiences with very different CPAs that you don't want commingled — CBO will optimise toward the cheaper one
Why CBO replaced it as default
Manual budget splits are guesses. CBO sees the live auction and shifts budget hour-by-hour to whatever's producing. For mature accounts running scaled prospecting in a single campaign, CBO usually beats ABO by 10-25% at the same total spend. ABO is the right tool for the job when the job is testing or compliance, not steady-state production.