This setting is only used when restoring a virtual server that does not currently exist on the system. By default, Virtualmin will either assign it the system's default shared IP address if it was on a shared IP originally, or use the private IP from the original system if it wasn't on a shared address.

This works fine if your are moving a domain between two systems on the same network, but if not the restored domain is not likely to be contactable via the network. To avoid this, select the Virtual with allocated IP or Virtual with IP option in this field, and Virtualmin will either select a free IP for you, or use the one you enter.