Deal ID, or deal identifier, is the unique number of an automated ad buy. The private marketplace is more accessible to inventory with a bidding mechanism, letting a limited number of brands/advertisers enter the market competing for the inventory. While deal ID, in understandable explanation, is a unique contract number for each of the deals made between advertiser/brand/agency and publishers. It is a parameter attached to the ad impression request. Since, both the publisher and advertiser pre-negotiate the specifics of the campaigns, it means better revenue for publishers and higher ROI for advertisers.