The network discovery is performed on the hosts entered in the Include tab (Scan -> Settings -> Hostname /IP Address Range ). The network discovery option shows you which IPs provided are live before the scan is run (which is a good way to know which IPs you should scan without consumption of your license). This information can be exported as a CVS file.
When providing a range of IP's(e.g. 192.168.1.1-100) to the scan include box a network discovery will be done automatically. In order to run it manually go to Other scroll down and click on the Perform Discovery button. After doing so you can decide which Ips to insert in your scan range and run.
After you click Perform Discovery, navigate back to the "other" tab and click "export". This will download the IPs as a CVS file.