The SET CHANNEL command can be used to open a document or port on a given machine. Because SET CHANNEL lets you interact with the open channel via commands such as SEND PACKET, RECEIVE PACKET, etc only one port can be open at any given time. Whichever port is open will respond to the send and receive commands.

But by calling SET CHANNEL in multiple processes each process can have a separate channel open. In this way you can interact with multiple serial ports and/or documents via the SET CHANNEL command.