A session is created' when a user opens a document for collaborative editing. The session is alive as long as there is at least one user still working on the document. When everyone in the session has left (by closing their browser tabs or exiting the app, for example) the session 'ends'. While a session is alive, users joining and leaving are not counted towards the session quota. For example, say you have 2 users editing the same document, if one user is on his/her mobile device and gets disconnected and then later reconnects during a session, we still count that as a single session as long as the other user remains in the session.