A fallback or passback code is a script that will be loaded if Viralize is not able to fill a certain ad request. You can specify a tag to be loaded from any third-party source. Also referred to as passback.
Viralize Ad Units can be set up with a fallback code so you can play other networks' ads when no video is available.
If you have a third party code, set a custom Fallback from the Ad Unit Edit section.