This GPLT graphics process handles all the graphics processing including mapping, symbol drawing, and coordinate transformations. When the graphic is ready to be output, a third process called the device driver starts up.
Message queues behave like mail boxes.
One process leaves a message in the mail box for the other process. Upon receipt of a message, the other process acts on the information in the message, and perhaps sends a message back to the first process, or to another process via another mail box. This functionality allow you to construct overlays, and well as multi-paged Postscript files. The GPLT graphics process and the device driver will continue to run in the background of your computer until you tell them to go away.