Talk:Viewable impression

"Viewable"
wish I could clearly understand wht viewable means here. The first sentence defines Viewble Impressions in terms of "viewability" without clarification. Isn't there a simple, clear first approximation of wht viewable means?

"Other companies to offer viewable impressions include"
is this list helpful? It could include hundreds of other companies and there seems to be little reason why these companies are listed and others are not.