Private marketplace (PMP) allows buyers and publishers to connect their sources to PX invisibly from other platform participants and have one centralized place for all their sources.
This helps to keep direct relationships between buyers and publishers separately from public traffic and at the same time use all features provided by the PX platform like source management, sales funnel, centralized billing, and extensive reporting.
As soon as publishers are connected to Buyer PMP account, all their sources become private, meaning that they start selling leads or calls only to campaigns of the buyers connected to the same PMP, and their leads/calls don't participate in the open exchange.
Publisher PMP account includes an option to sell leads to both buyer accounts connected to the PMP and to Open exchange. For more details, follow the article: How to enable Open Exchange traffic for Publisher PMP