GAM Backfill Viewable Impressions % is a Metric in Google Analytics under the DoubleClick for Publishers Backfill section
GAM Backfill Viewable Impressions % Definition: The percentage of backfill ad impressions that were viewable. An impression is considered a viewable impression when it has appeared within the users’ browsers and had the opportunity to be seen. If AdSense and Ad Exchange are both providing backfill ads, this metric is the weighted average of the two backfill accounts (DFP linking enabled and Hide DFP Revenue not enabled).
