How can brands purchase Ad Inventory with programmatic?

Advertisers may buy ad inventories in different ways, including:

RTB or real-time-bidding is a process of buying inventory through open auction in real time. Using this method of purchasing ad inventory, advertisers can buy per impressions, per click and so on to show their advertisements only to the right audience. Since the process is automated and fast, ad impressions are sold and bought with help of specialized platforms — DSP, SSP, Ad Exchange. 

Programmatic Direct is the media buying process when advertisers buy ad impressions in bulk at a fixed rate and definite place instead of going through RTB. In this case, media buyers get a guarantee of displaying their ads in a place where they want to advertise. However, there are several cons of that model such as higher prices, high competition, and opportunity to miss target audiences. 

Private Marketplace (Deals or Packages) is a closed RTB auction, participation in which is available only by invitation by publishers of a limited number of advertisers. This type of media buying allows advertisers to gain first access to a specific ad inventory before it’s made available to other buyers in an open auction. In this case advertisers may get high-quality premium ad inventory with a high level of brand safety.