A publisher can create rules that will affect the bidding process of its inventory. For example, a publisher can create a rule specific to a certain demand partner and set a price floor value. This will ensure that the publisher receives certain minimum revenue from that demand partner when the demand partner bids for the publisher's inventory.
Demand partners, typically DSPs, connect to the PubMatic system using an RTB campaign. There are certain RTB partners to which the PubMatic system contacts to ask for the bid for an impression. Since there are multiple DSPs in the bidding process, they bid different amounts for the impression. Depending upon the maximum bid amount, the ad auction engine selects the winning ad network/DSP and adds this winner’s URL in the ad tag response back to the visitor’s browser.
RTB Rules Documentation