Live sessions
Live Sessions lists everyone online now: username, NAS, IP address, how long they have been connected and how much they have used. Routers report usage every two minutes, so the numbers lag slightly. You can disconnect a session from here.
If a router stops sending updates for 5 minutes (it lost power or its internet), its sessions are closed as stale. They do not count as online any more.
History and usage
Finished sessions are kept without a time limit. Each subscriber's page shows their current usage against their plan's limits, and their usage over time.