Client-managing Sessions
According to the XSMP protocol, a session manager is an arbitrary program that runs and controls the state of other applications. As a result, a client can itself be a session manager of other clients. For example, a mail client can start a text editor for the sake of writing an email, and behave as a session manager with respect of the editor. This way, if the mail client is closed and then restarted again, it can restore the state of the text editor.
Read more about this topic: X Session Manager