Final Draft’s Collaboration tool uses Amazon Web Services for the connection between users' computers. When a script is shared via a Collaboration session, it is not uploaded to or stored on an Amazon server, a Final Draft server, or any other third-party server. The script is only ever on the participants' computers (or his or her local storage) and can be saved at any time during the session with the regular File > Save or > Save As command.
The script is encrypted during transport and can only be decrypted by the client. Only the host is able to decide whether or not another client can connect to the same session.