If the server accepts the BIND, then a session is established. The PU 2.1 protocol does not state how the client knows which node contains the server program. In classic SNA, each display, printer, and application program has a Network Address number. Messages flow through the network from node to node based on the Network Address of their ultimate destination. When the BIND is processed, the Session ID is associated with a route from node to node through the network between the client and the server.
Messages flow on that session.
The Session ID is local to a particular pair of nodes. There is no way to determine from this local session ID where the message started or where it is going. At each point, the session ID only indicates the next node to which the message will be sent. In APPN, there are communication subsystems on each minicomputer. When a system comes up, its local subsystem establishes some utility sessions with the subsystems on adjacent nodes.