Revenue per eligible opportunity measures ad yield across all the moments where a placement could have served an ad, including the ones that never filled or never showed. That makes it harder to game than eCPM (which ignores unfilled requests) or fill rate (which ignores price).
It is the right lens for floor changes, network additions and waterfall-to-bidding migrations. If a change raises fill but this number barely moves, the extra fills are low-value.
Defining “eligible” consistently — per placement trigger, not per request — is what makes the metric comparable across tests.