NUL, AUX, CON, COM1-4, LPT1-3, and PRN are reserved words used for DOS Devices
These are specified in IO.SYS and date back from the early MS Dos days. Here is a brief list:
CLOCK$ - System clock CON - Console; combination of keyboard and screen to handle input and output AUX or COM1 - First serial communicationport COMn - Second, Third, ... communicationport LPT1 or PRN - First parallel port NUL - Dummy port, or the "null device" which we all know under Linux as /dev/null. CONFIG$ - Unknown
So just like any programming language has reserved words, CON is reserved for a console device used by dos system. Hope this helps!