After setting the 'admin' password via WebAdmin GUI during initial setup wizard, you can define passwords for system (console) users 'root' and 'loginuser' at the Management >> System Settings >> Shell Access >> Shell User Passwords section.
The 'admin' user is only used for GUI administration via WebAdmin, while 'root' an 'loginuser' are real system (console) users that do not exist within the GUI.
If you have physical access to the console of the machine, you can also set 'root' or 'loginuser' password by simply trying to login for the first time. You are then asked to set the password.