Viewable impressions per ad start according to the MRC. This metric is one of the key KPI measured by advertisers. A low viewability is affecting your fill rateLearn more : MRC : An ad impression will be considered as a viewable one when the ad is contained in the viewable space of a web browser’s window. If the ad is getting displayed outside the viewport of a browser, then the ad impression didn’t have the chance to be seen by a user.