When a session is started it's state is set to active. It can become inactive in one of the following ways:
User Manager receives accounting-stop message;
Customer closes session manually in the web interface. The option "Close" is available for the active-session table, on the status page;
An active session is closed when the same router asks to start a new session with the same accounting-session-id.
If the router hasn't sent accounting-stop message the session may remain active even if it should have closed much sooner. Such sessions can be closed manually.