Glossary
What is Passback?
Also called: passback tag
A passback hands an ad request that one demand source cannot fill to another source, so the slot gets another chance to show a paid ad.
हिंदी में पढ़ेंIn a waterfall setup, the ad server offers an impression to demand partners one at a time, ranked by their past average prices. When a partner cannot or will not fill it, for example because no bid clears its floor, it returns a passback: a tag or signal that sends the request on to the next source in line.
Each passback adds another call, so the reader waits longer before an ad appears. The chain also stops at the first partner that fills, so a partner further down that would have paid more is never asked. If nothing paid serves at the end, a house ad (your own promo, worth nothing) can fill the slot, or it stays blank.
Header bidding avoids this by asking every partner for a bid at the same moment, before the ad server decides. If your partners sit in a ranked list with fixed prices, you are running a waterfall, and passbacks are part of how it works.
Example
A sidebar slot sends 10,000 requests to Network A first. A fills 7,000 and passes back 3,000 to Network B, which fills 2,000 and passes back 1,000. Those 1,000 get a house ad. Paid fill is 9,000 ÷ 10,000 = 90%, and 3,000 requests waited for at least one extra call.
Why it matters to publishers
Every passback costs time and can leave money on the table, because the partner willing to pay most may never be asked. Knowing where passbacks happen shows where a waterfall is still costing you.