You can invite a set of buyers to bid on a portion of your inventory via an auction with Private Auctions. You specify a minimum CPM floor price on a per-buyer basis, and that buyer must exceed the floor price to be eligible for the auction. The winner is determined using the same auction model that applies to the Open Auction.
Private Auctions hold general information about the portion of your inventory you want to make available for an auction. Once you start a Private Auction, you can invite buyers to participate. Each invite becomes a Private Auction deal with a single buyer.