... Default Tag:
If no impression can be found for an ad opportunity, a default tag may be served that triggers a second round of decisioning usually (but not always) in another ad serving system.to pass the impression to another system to see if demand can be found there (this happens in the case of Display Advertising Glossary) or to simply pull a creative from another ad server. Sometimes called "Passback." Demand:
Advertising demand; entities that wish to buy ad space and display creatives.