Target ACOS
The efficiency you are aiming for. Every bid decision is measured against it.
The single most consequential setting. It can come from three places, in increasing specificity: an account default you set, a per-group target you set, or a target derived from your product margins.
Whichever resolves, the winning target and its derivation are frozen into every proposal as evidence — so a decision can always be traced back to the number that justified it.