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.
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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.