How it works
It is counted on the products that really do get impressions, not on the ones uploaded into the feed. The gap between “uploaded” and “showing” is the most common place where revenue goes missing.
Example
The catalogue holds 3,000 products; 1,200 of them got impressions over the month.
Coverage = 1,200 ÷ 3,000 × 100% = 40%. The other 1,800 took no part in advertising at all.