Served means the ad actually rendered — that's the billable event, not the ad request.
We bill on served impressions, and a served impression means one thing: the ad
actually rendered in the browser. The tag fires a served beacon at first paint, and
that beacon — nothing earlier — is what bills.
When a page asks us for an ad, we run the auction and return a winner. That request is a fill — it tells you whether inventory was available, and it's the number behind your fill rate. It is not a billable event. Plenty of things can happen between "we returned an ad" and "the ad rendered," so we wait for the render before anyone is charged.
An impression that renders below the fold still counts as served. "Served" is about the ad being painted, not about whether a person scrolled to it.
We also record a viewed event using the IAB standard — at least 50% of the ad's pixels
on screen for at least one continuous second. Viewability is reported from day one so both
sides can see it, but in the current model it is never billed. Only served bills.
Advertisers pay for ads that rendered, not for requests that may have gone nowhere. Publishers earn on the same event, read from the same log. There is one source of truth, so your dashboard, your reports, and your balance always agree.
For the exact filtering rules that decide which served events are billable, see How we count & filter traffic.