A passback is a mechanism used in the context of online advertising to enable an ad server to serve an ad from a different source or network if it is unable to serve an ad from its own inventory. A passback is often used when an ad server is unable to fill an ad request due to a lack of available or eligible ad inventory. In this case, the ad server can use a passback to serve an ad from a different source, such as a backup ad network or a house ad, in order to fulfill the ad request. Passbacks can be beneficial for advertisers, as they can help to ensure that their ads are delivered to users in a timely and efficient manner. Passbacks can also be beneficial for publishers and platforms, as they can help to maximize the value and revenue of their ad inventory, and to provide a better user experience for their audience.