What is a fallback URL?
It’s the backup URL, that is used when the traffic cannot reach the main page for some reasons (filter, cap). Instead of showing the visitor not beautiful “disabled” screen, you can send those clicks to a backup offer or even google page.
You can set your fallback URL in Tools -> Default fallback URL section.
You can place any link on which you want to redirect the traffic from your Campaign as a Default fallback-URL.
Also, you can use Sub ID parameters to send some important data if necessary. These parameters are to be used the same way it is done on a campaign level.
We suggest you use some additional offer and add its link as a default fallback URL, it will help you not to lose your traffic.