What each one is actually doing
ACoS = ad spend / ad sales. It answers: for every dollar I spent on this campaign, how many cents of ad-driven revenue came back? Useful for keyword-level optimization, daily bid management, A/B testing creative. Limited because it ignores the halo — the search you bid on may have driven an organic conversion later, an upsell to another SKU, or a Subscribe & Save signup. None of that shows in ACoS.
TACoS = ad spend / total sales. As organic sales grow relative to ad-driven sales, TACoS falls — even if ACoS is flat. That's the indicator a brand is building durable demand rather than renting traffic.
Why this matters for the JBP
When you commit a media investment number in the JBP with Amazon, you're committing against a result. A campaign-level ACoS target is the wrong unit for a multi-quarter commitment. A TACoS trajectory is the right one — it ties media spend to brand-level outcomes and lets you negotiate co-funded commitments against share-of-search and new-to-brand growth, not just incremental campaign ROI.