A non-guaranteed fixed is an exclusive, advertiser-to-publisher relationship for programmatically purchasing inventory in brand-safe environments. Additionally, it provides advertisers with first-look access to custom, non-guaranteed inventory at a fixed CPM.
If a non-guaranteed deal's terms are met (which generally refers to CPM, targeting, ad-related requirements), the impression goes to the non-guaranteed deal advertiser first.
If the deal terms are not met, the impression goes to the open auction or a non-guaranteed auction (if the publisher is participating in one). Note that the advertiser is excluded from bidding on this inventory in the open auction.