If I remember correctly, "PR#6" was basically just a command that executed the code at $c600. Each peripheral card had a 256-byte address space which was mapped into $cx00 where x was the slot number. Slot 6 was the standard slot for the disk controller -- it may even have been wired in, in later machines. So "PR#6" basically executed the firmware boot loader for the floppy drive.
What I found weird was that I remembered the command, but not the technology it came from or exactly how I used it. So I'm Googling around trying to figure out when and why it was important. For some reason I remembered it as "Print #6".
Old age and forgetfulness is going to be really strange for a whole generation of computer nerds. So many different technologies, commands, and paradigms all jumbled up.