What is difference between impressions and viewable impressions?
Active View measurement terminology
Measurable impressions: Impressions that can actually be measured with Active View technology; a subset of all eligible impressions. Viewable impressions: Impressions that were considered viewable based on MRC standards; a subset of measurable impressions.
What is viewable impressions in Google Analytics?
What is a viewable impression and viewability? An impression is considered a viewable impression when it has appeared within a user's browser and had the opportunity to be seen. ... For larger ads (those greater than 242,000 pixels), 30% of the ad's pixels are visible in the browser window.