A session is a group of processes that have an instance of the ToolTalk message server in common. When a process opens communication with the ToolTalk service, a default session is located (or created, if a session does not already exist) and a process identifier (procid) is assigned to the process. Default sessions are located either through an environment variable (called "process tree sessions") or through the X display (called "X sessions").
The concept of a session is important in the delivery of messages. Senders can scope a message to a session and the ToolTalk service will deliver it to all processes that have message patterns that reference the current session. To update message patterns with the current session identifier (sessid), applications join the session.