Anyone know the difference between "filled" and "impressions" on the Facebook Audience Network?

Anyone part of the Facebook Audience Network beta for mobile ads? In the reporting they show "requests" "filled" and "impressions" - screenshot here: https://www.dropbox.com/s/nwaj5vlox8tr6u2/Screenshot%202014-10-29%2018.31.02.png?dl=0 But they don't seem to track "fill rate" like normal ad networks do, they track it on what they call "impressions" not on what they call "fill". Anyone know? There is no support link for FAN developers.