Target ACoS
Set the ACoS you want per product and see bid suggestions.
Target ACoS lists your enabled ad groups, biggest spend first, so you can set the ACoS you're aiming for in each.
Setting a target
Type the percentage in the ad group's Target ACoS box: 25 means 25%. It saves when you leave the box and says
Saved. Clear the box to remove the target. The filter box finds an ad group by its name or its campaign's.
Next to each target you see the ad group's spend and ACoS over the last 30 days (Amazon's usual 7-day attribution, as in Seller Central), and the rule sets that run on it.
What uses it
Only campaign rules whose condition is ACoS vs. Target ACoS, for example "lower the bid when ACoS is over the target + 10 points". An ad group without a target is skipped by those rules. On their own, targets don't change anything in Amazon.
Not sure what to aim for? The breakeven ACoS column on Products is the point where a product's ads eat all its profit. A target below it keeps ads profitable.