When an ad slot does not receive any bids above the set price floor (typically $0.10) an impression is considered Remnant Inventory. Instead of simply going unfilled, remnant inventory passes back to a set of alternative demand, outside of the header biding auction, as a final effort to fill the impression.
Remnant provides an incremental source of revenue since the impression would've otherwise gone unfilled. A high % of remnant impressions is an indication that your price floor is too high OR that a high % of your traffic is not of value to advertisers.
If Remnant does not fill the impression, a blank will show in the ad slot. The impression will still be counted as remnant as we do not have visibility into whether remnant filled the impression or not. Similarly, remnant impressions are estimated at a conservative $0.01 CPM. The actual eCPM tends to back out to about $0.05.