A session starts when a message is received.
There might be any number of messages in one session.
If during 15 minutes there are no messages then the session will be closed. So if a person sends a message again the new session will be started.
With each new person’s message session renews.
Opening event is available in web widget. It is automatically triggered when the web widget is opened.
Opening event doesn't start a session.
The session will be started when a person replies to a message that was sent by the opening event.