What is open auction?
The open auction is the public tier of programmatic, where any buyer the exchange admits can bid on an impression without a prior agreement. In OpenRTB it is the default: a request with no Imp.pmp, or with Pmp.private_auction at 0, accepts bids that carry no deal ID. The floor comes from Imp.bidfloor and the pricing rule from BidRequest.at.
Also called: open market, open exchange, OMP, open marketplace.
How it works
An impression can be in the open auction and in deals at the same time. When Imp.pmp lists deals and Pmp.private_auction is 0, the exchange accepts both deal bids, which echo a Bid.dealid, and open bids, which leave it empty. Exchanges often give deal bids priority, and an open bid that loses to one can receive loss reason 103 (Lost to a Bid for a Deal). Setting Pmp.private_auction to 1 closes the open tier for that impression.
Open-auction pricing follows the request-level rules. Imp.bidfloor and Imp.bidfloorcur set the minimum, BidRequest.at says whether the winner pays its own bid or a second-price amount, and BidRequest.wseat or BidRequest.bseat can still restrict which buyer seats take part. Buyers face the most competition here, so bid shading and supply path choices have the biggest effect on what they pay.
Two mistakes are common. Sellers set Pmp.private_auction to 1 while still expecting open demand, and fill drops to the deal buyers alone. Buyers return a Bid.dealid the request never offered, expecting deal treatment, and the exchange either rejects the bid or prices it as an open bid. Reading the pmp block of a real request before launch catches both.
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Related terms
Every term is in the glossary.