[mvkg]

What were the PRN and AUX files used for?

[Karellen]

PRN was for the printer.

AUX is for "auxiliary" (aka "misc."/"other") device - i.e. any other bit of unspecified random hardware you'd managed to hook up with a custom driver by yourself.

[mkup]

AUX is just a synonym for COM1, i.e. first serial port in DOS/Windows.