source address is optional, you can leave it empty. The virtual server accepts clients only from one of these IP addresses, and by default F5 accepts all IPs (0.0.0.0/0). if your VS should only be reached from a specific range or IP you can configure it in this section.
Regarding "Destination Address/Mask" this is the listening IP of your VS.
This IP will be propagated via gratiutous arp... to inform the network elements around that this IP is carried by F5 ...