Measurement
Incrementality
Incrementality is the share of conversions that happened because of a marketing activity and would not have happened without it.
It is the question attribution cannot answer. Attribution assigns credit for conversions that occurred; incrementality asks which of them were caused. A retargeting campaign shown to people already intending to buy can post excellent attributed ROAS while being close to entirely non-incremental.
Measuring it requires a holdout — geographic, audience-based or time-based — where the activity is withheld and the difference observed. That is uncomfortable, because it means deliberately not advertising to some people, and it is the only method that produces a causal answer.