Open exchange

Amazon describes the open exchange as a public real-time bidding marketplace “designed for scale and efficiency,” where publishers make inventory available to all advertisers simultaneously — supply Amazon calls “Massive,” non-guaranteed, spanning premium placements to broad reach, and “typically the most cost-effective buying method.” The pricing line you budget against is Amazon's own: variable CPMs apply to open-auction bidding and fluctuate with market demand, while fixed CPMs apply to deals. Keep the two deal types apart — a private marketplace is a pre-negotiated floor CPM and still an auction, while programmatic guaranteed is a fixed pre-negotiated CPM with no bidding at all.

The failure mode is budget pressure meeting effectively unlimited supply. Amazon's published default bid strategy — “While spending full budget, maximize performance” — puts budget delivery ahead of performance, and max average CPM is documented only as a limit you set manually; Amazon publishes neither how bids behave beneath it nor whether clearing is first-price or second-price. Build the report, not a theory: split open exchange into its own ad group with its own max average CPM, and read performance by supply source rather than the ad-group blend.