When you create a project, you can define an optional network range that sets the scope of the project. The network range defines the addresses that Metasploit Pro uses to automatically populate the target addresses for discovery scans and Nexpose scans. It also defines network boundaries that Metasploit Pro can enforce for the project.
You do not need to set the network range unless you want to enforce network boundaries. If you choose to enforce network boundaries on a project, Metasploit Pro uses the network range that you define for the project.