This controls whether we include this interface in LSPs even if it is not running the IS-IS Protocol. Marking a circuit passive lets the router advertise that interface's connected prefix inside its own LSPs without ever forming an actual adjacency or sending Hellos over it, the classic use case being loopback interfaces or stub links where no real neighbor will ever answer. For example, setting this true on a router's loopback interface ensures that /32 loopback address gets advertised into IS-IS reachability without the router wastefully trying to send Hellos out an interface that has no neighbor to receive them.