Private marketplace includes both Private Auctions and Preferred Deals. These options give you a way to interact with a select group of buyers, or even a single buyer.
Private Auctions occur when one specific publisher holds an auction with a group of select buyers. Unlike the Open Auction, a private auction gives a select group of buyers priority to inventory before it becomes available in the open marketplace. For example, if a publisher decides to sell most of their inventory anonymously in the Open Auction, Private Auctions can be used to sell some fully branded inventory to a select group of buyers.
Preferred Deals are an option that bypasses auctions completely. These deals take place when one publisher makes a deal with a fixed-price CPM, to one specific buyer or advertiser. Generally speaking, preferred deals tend to warrant higher CPMs because the publisher makes one specific buyer privy to their premium inventory before anyone else.