What is a private marketplace (PMP)?

Private marketplaces (PMPs) are customized, invitation-only RTB marketplaces where publishers make their inventory and audiences available to a select group of buyers. While there is still an auction in a PMP, the auction is limited to buyers who have been invited to the auction. 

Publishers offer premium inventory and page placements, high-impact ad units, and the usage of first-party data through PMPs, and advertisers appreciate PMPs because they alleviate concerns about things such as brand safety, transparency, and inventory quality.