Traffic & Channels
Ad Stack
Multiple ad units layered within a single placement, often used to maximize fill.
An ad stack is multiple ad units layered within a single placement, commonly used by publishers to maximize fill rate and revenue — if one network fails to fill the slot, the next layer in the stack serves instead. In the waterfall era stacks defined mobile monetization; header bidding flattened but did not eliminate the pattern.
Buyers mostly feel ad stacks as inventory-quality noise: your impression may render in a stacked slot beneath or behind other units, tanking viewability while the CPM stays the same. Aggressive stacking also flags low-quality publishers whose whole business is squeezing slots. If a placement's impressions are huge but engagement is near zero, a stacked or hidden ad unit is a likely explanation.
In buyer speech
“Impressions on that zone tripled overnight with zero clicks — smells like the publisher stuffed our unit into an ad stack.”