A passback is a fallback option for third party ad serving setups. There are instances when your third party ad server cannot find an applicable ad to serve. This can happen for a number of reasons, including pricing in real time bids, geographic restrictions, and targeting requirements. A passback ensures that your setup has a pool of backup ad items to serve in such instances.

A passback gets its name from the fact that the third party server will pass the ad request back to AdButler if it does not find an applicable ad.

To set up a passback in AdButler, you must dedicate two zones to your setup. The first zone is the standard one that contains the third party ad tag. The second zone contains your backup ads from AdButler, another third party ad server, or both. You will then provide the zone tag of the backup zone to your primary third party ad server.

This is a simplified breakdown of the passback at work:

1. The zone tag in the publisher's site makes a request to AdButler.
2. AdButler returns the primary third party ad tag.
3. The zone tag requests an ad from your third party ad server.
4. The third party ad server cannot find a suitable ad, so it instead returns the zone tag of the backup zone.
5. The zone tag requests an ad from AdButler using that backup zone tag.
6. AdButler delivers an ad to the publisher's site.

As previously stated, you can also have another third party server as your backup. You can even have multiple passbacks in a chain.