[Term in DART for Publishers] System default The 1 x 1 pixel transparent GIF file, sometimes called grey.gif, that DART serves when there are no other ads available, including default ads in DART for Advertisers or house ads in DART for Publishers. The ad slot in the webpage or message is displayed as a blank space, not as a broken image.
Unfilled impressions An ad request made by your GA_googleAddSlot calls that didn't return an ad. Unfilled impressions are tracked in your inventory reports. Learn how to generate an unfilled impressions report)