Depending on the SSP used, a Deal ID is a numerical or alphanumerical identifier passed in the actual the bid request to DSP. The Deal Id (when mapped in both SSP and DSP) enables the buyers DSP to identify private marketplace inventory and bid accordingly to the rules set in the DSP.
One to One Deals
Most Deal IDs operate on a strict One to One basis. That is; a Deal ID is setup for a single DSP and enabled for a seller and buyer only to transact on. An example of One to One Deals would be Healthy Ads (as the Seller) and ULTRA Mindshare (as the Buyer) transacting on a specific Deal ID.
One to Many Deals
One to Many Deals are slightly different, whilst limited to a single DSP, the seller enables that any buyer using the DSP can access the deals. Sometimes One to Many Deals are referred to as Platform Deals or Pre Negotiated Deals. An example of a One to Many Deal would be Healthy Ads (as the Seller) and the TradeDesk (as the DSP) and the deal is used by an Agency transacting on a specific Deal ID.