A slot comes free.
A screen runs a loop — say ten slots of ten seconds. One of those slots is unsold, or has been released. The player knows this before the loop reaches it.
10s slot · 6 loops/minpDOOH Pro Academy · explainer
Programmatic out-of-home is usually explained as a diagram with seven boxes and an arrow. That tells you the parts, not the sequence. This is the sequence — what actually happens, in what order, in the roughly two hundred milliseconds before a frame changes.
A screen runs a loop — say ten slots of ten seconds. One of those slots is unsold, or has been released. The player knows this before the loop reaches it.
10s slot · 6 loops/minThe supply platform turns that empty slot into something a buyer can value: where the screen is, how big it is, what the audience looks like at this hour, what content policy applies. Location and time are doing most of the work.
geo · daypart · audience indexThat description is broadcast to demand platforms as an OpenRTB request. Out-of-home carries fields the web never needed — screen dimensions, expected dwell, how many people the slot is likely to reach rather than one person on one device.
OpenRTB 2.6 · DOOH extensionEach demand platform checks the slot against its campaigns. Does this location match the audience the advertiser bought? Is there budget left today? Is the weather trigger satisfied? Most say no. A few return a price.
typical response < 120msThe auction settles and a single creative is selected. The losing bids cost nothing and are never spoken of again. This is the part that genuinely resembles digital display — and the only part.
second-price · floor appliedThe creative is delivered to the player and shown on the physical screen at the scheduled second. No cookie, no device, no click. A person walks past a wall and sees something.
1 play · n impressionsPlayout is logged and reconciled against an audience measurement, so one play becomes an impression figure the buyer can compare with everything else they bought. That reconciliation — not the auction — is where out-of-home is still arguing with itself.
proof of play → impressionswhy it matters
Most explanations of programmatic out-of-home stop at step five, because steps one to five are borrowed wholesale from digital display and are therefore easy to describe. The interesting problems live at either end: describing a physical place well enough for an algorithm to value it, and turning one play on one wall into a number a media buyer can compare against everything else on the plan.
Get those two right and the auction in the middle is plumbing.