They are implemented as standard UNIX print queues and can be accessed with the UNIX lpr command. Files written to a queue must be in the correct format (see ESO WWW page http://http.hq.eso.org/garching-info/computing/devices.htm)
In the case of Colour Slide the MIDAS command COPY/DISPLAY SLIDE can be used. This command will both create the PostScript image file and submit it to the slide queue.