A PMP (Private Market Place) allows publishers to control the price and the advertisers that run ads on their websites and apps.
In the PMP only the selected advertisers can run their ads and publishers establish a floor price for that. Imagine that there are 4 advertisers for the same PMP, the auction will happen only between those advertisers.
The premium publishers, as they can establish high floor prices, normally they don't open their inventory on Open RTB, as with the PMP's they sell high price and they keep the relation with the advertisers.
How to set up a PMP? 1- Identify the placement and the publisher where you want to do the deal. Also, you should know the ad sizes. 2- Contact the publisher and tell him the DSP you are using, as well the deal you want to establish. 3- The publisher will inform you of the floor price, the deal id, and the ssp. 4 - Then you just need to do the deal setup in the DSP and your deal should work as soon as you launch a campaign on it.