Ad opportunities (also known as "historical ad opportunities") are the number of times it was possible to have delivered an impression in the past, even if there was no actual impression for an ad request.
How ad opportunities are calculated
The "Impression rate" multiplier accounts for the fact that Ad Manager counts ad impressions using the downloaded methodology and mobile app impressions when the creative is displayed on screen. Using this multiplier with "Unfilled impressions" provides a more accurate picture of missed impression opportunities.
For a segment of traffic that has 95% of served impressions that are also downloaded impressions, the impression rate will be .95.