ACoS measures ad spend against ad-attributed sales only. Cut spend and ACoS improves — while the organic rank those ads were feeding quietly erodes. Sellers optimize themselves into decline every day this way.
TACoS divides ad spend by total sales, capturing the organic halo that advertising creates on Amazon. A rising TACoS with flat sales signals dependency; a falling TACoS with growing sales means your ads are building durable organic rank.
Set TACoS targets by product lifecycle — launch products run hot, established listings run lean — and let ACoS float. Your bids start serving the business instead of the dashboard.