CPR ('units to rank') is the number of units a listing needs to sell — typically measured over an 8-day window — to reach page 1 of Amazon search results for a given keyword.
It's a cost-of-ranking metric: a lower CPR means fewer units need to be sold (often via PPC or promotions) to break onto page 1, so the keyword is cheaper and faster to rank for. Used alongside Market Availability Metric and conversion rate in the High-Conversion Keyword Selection Strategy to select keywords that are winnable, not just high-volume.
CPR is calculated over an 8-day window: it estimates how many units a listing needs to sell in those 8 days to reach page 1 for a given keyword. Sellers compare CPR across candidate keywords or launch strategies and favor whichever route requires fewer total units (i.e., is cheaper and faster) to hit page 1.