A Server session object. from OPCUA Spec 1.02:

• Sessions are created to be independent of the underlying communications connection. Therefore, if a communication connection fails, the Session is not immediately affected. The exact mechanism to recover from an underlying communication connection error depends on the SecureChannel mapping as described in Part 6.
• Sessions are terminated by the Server automatically if the Client fails to issue a Service request on the Session within the timeout period negotiated by the Server in the CreateSession Service response. This protects the Server against Client failures and against situations where a failed underlying connection cannot be re-established.
• Clients shall be prepared to submit requests in a timely manner to prevent the Session from closing automatically.
• Clients may explicitly terminate Sessions using the CloseSession Service.
• When a Session is terminated, all outstanding requests on the Session are aborted and Bad_SessionClosed StatusCodes are returned to the Client. In addition, the Server deletes the entry for the Client from its SessionDiagnosticsArray Variable and notifies any other Clients who were subscribed to this entry.