both can be installed internally or externally, to the internal or external DB25 parallel port
the Digi-Gen has to be connected to the Amiga's parallel and joystick ports
when the the digitizer is installed internally, the external DB25 port acts as a parallel passthrough port - the DI/PR switch selects between the digitizer and passthrough
when installed outside, the digitizer is fed with composite video signal through the DI output connector (RCA)