The private marketplace (PMP) gives you greater access to inventory from premium publishers. PMP deals let you bid on impressions that might not yet be available in open bidding, and can give you access to exclusive, less competitive auctions. Some PMP deals may provide specialized targeting that is not available otherwise (for example, targeting users interested in wedding recipes or the next Olympics).
Platform deals are available to all the DSP users. They can be used in your tactics right away—no extra negotiation is needed. The the DSP team has negotiated these deals on behalf of all the DSP users. See Platform Deals for details.
If you've negotiated your own private marketplace deals with a publisher or an exchange, you enter these deals into the DSP and manage them through My Deals.