With a Passback tag, the 3rd-party server will send your request back to AdButler if there is no applicable ad in its service. 3rd Party Ad Servers with real time bidding often have pricing or geographic restrictions that can leave you with a blank ad when no ads are available. Passbacks ensure that you will always be able to serve a fallback.
To use Passbacks tags with AdButler, you'll want to create 2 zones: One to contain your 3rd Party ad tag, and another to contain your backup ads from AdButler and/or another 3rd Party ad server. You'll give this backup ad tag to your 3rd Party ad server to use in case no 3rd party ads are available. You can have multiple Passbacks in a chain.
The picture below is a breakdown of the process. Firstly the ad tag makes a request to AdButler and returns the initial 3rd party ad server (script/tag). The ad tag then makes a request to the 3rd party ad server for an ad. If no ad is available the 3rd party ad server returns the backup tag. The ad tag then makes a request to AdButler using that backup tag, and an ad is returned.