This enables or disables the use of a globally-shared set of TACACS server definitions, presumably as an alternative to server lists configured independently per feature or context. An admin managing many devices would typically want this on so a single shared TACACS server list applies consistently everywhere, rather than risking drift where one device's authentication server list and another device's accounting server list point at different, inconsistent sets of TACACS servers. For example, enabling this on a fleet of switches ensures that updating the global server list once propagates that server to every context on the device, rather than requiring separate updates to several per-function server lists.