ROAS (return on ad spend) is sales attributed to ads divided by ad spend. $1,000 in sales from $250 of spend is a ROAS of 4, sometimes written 4x or 400%.
ROAS and ACoS are the same number
ROAS is simply 1 divided by ACoS. A 25% ACoS is a ROAS of 4. Amazon's Campaign Manager shows both; agencies that quote ROAS are usually coming from Google or Meta advertising, where it is the standard. On Amazon, ACoS is the more common language, and TACoS is the one that matters for the business.
When ROAS is useful
ROAS reads well when comparing channels, for example Amazon Sponsored Products against a Google Shopping campaign that sends traffic to Amazon. Inside Amazon, treat it exactly as you would ACoS: judge it against the product's margin, not against a generic benchmark.