New-to-brand reach — whether a DSP ad reaches consumers who have not previously engaged with the brand — is the recommended metric for judging Amazon DSP (Demand-Side Platform) campaign success, rather than short-term profitability or ROAS.
The reasoning: DSP campaigns that only retarget existing engagers duplicate what cheaper Sponsored Display already achieves, so evaluating DSP on immediate profitability undersells (or wrongly kills) campaigns that are actually doing their real job of top/mid-funnel expansion. If a DSP campaign's new-to-brand numbers are low, redirect that budget to Sponsored Display instead of dropping DSP altogether.
The new-to-brand metric also appears in Brand Analytics generally (not only DSP reporting) and in Ad Console campaign reports. Track its growth percentage over time as a leading indicator of acquisition, and cross-check it against new-to-brand campaign ROAS/ACOS in Ad Console — see LACoS (Lifetime ACOS) and Lifetime TACOS for why acquiring new-to-brand customers pays off for consumable brands.