Some restricted services of the University or the Staats- und Universitätsbibliothek can only be used if you are on campus, so that the computer you use has an address from the area assigned to the university.
If you want to access these services from home or on the road with an account from the Zentrum für Netze, you must install a so-called VPN client software (AnyConnect or OpenConnect) on your computer and establish a connection to the campus with the VPN client before each access to the restricted-access services. This means that you are "virtually" on campus and can use the usual services.
Only current employees and students of the university may use the VPN service. For reasons of licensing law, use by, for example, alumni is excluded.