Depends on what type of ads are running at the moment, and who you are working with I suppose. As far as me and my crew can figure out, it seems if you are working with Admob then Admob will display all of their ads they have available that are paying per impression, when those run out it seems they go to the highest bidders on cost per click. Still working on trying to figure it out exactly, but that is what we have deciphered so far.