You can manage multiple networks from a single Studio account.
This is not common.
The Account pulldown menu lists all user names associated with the Studio account to which you are currently logged in and the networks with which they are associated. Use this pulldown to select the network you wish to manage.
While most users will have permissions for only a single network, some Studio users may own or moderate multiple sites. The Studio account allows them to log in with a single username and password, then switch between multiple site access levels.