What are eligible impressions in Google Ads?

Google says impression share includes all auctions where your ad showed, and all auctions where your ad is competitive enough to show. Its example: it could include auctions where your ad could show at twice its current bid, but could exclude auctions where your ad is estimated to need a 1000% bid increase in order to appear. It adds that changes to your bids, quality or Google's ad systems may change the set of auctions in which the system estimates that you were competitive.

The formula rearranges: a period's eligible impressions are its impressions divided by its impression share, written as a decimal. That is our arithmetic on Google's formula, and it holds only where the two columns describe the same traffic. Google says search partners won't be included in impression share data, that on Shopping campaigns only your first impression for each search will qualify for impression share, and that Performance Max impression share is impressions from Search + Shopping over total eligible impressions from Search + Shopping. So we do the division inside one Search or Display campaign, on an impression count that leaves search partners out.

Our reading, not Google's: the pool grows and shrinks with how many people search, but it also moves with your own bids, targeting and quality. A change in it reads as a change in demand only between two periods in which none of those changed on the campaign, and even then Google's own systems can move it.