Impressions are not about who has seen your ad. Instead, they‘re about the potential that your ad has to be seen. So you can’t really assign a reliable, quantitative weight to these metrics.
This metric isn’t based on an action, but instead acts as a valuable part of calculating other critical metrics that will lead to better campaign performance.
The two types of ad impression are:
#1: Served – when served content is transmitted it counts as an impression
#2: Viewable – when more data is used to understand if a user saw the ad content or not
Served impressions are unreliable, while viewable impressions are more accurate. The most vital of the two are viewable impressions because they provide data analysts with actionable performance data for campaign improvement.