Target source
Where your governing ACOS number comes from: account-wide, per group, or from margins.
Manual account-wide — one number for everything. Simplest, and blunt if your catalogue has varied margins.
Manual groups — a target per product family. Better, and what most accounts should run.
Margin-weighted — derived from each product's actual break-even. Most accurate, and it unlocks once cost data covers enough of your spend.
Group overrides always beat account defaults. Your Review decisions always beat everything.