Gregory's Server"End of Medium" basically meant "Hey, stop trying to read anything from this device!" It was there for punch cards, paper and magnetic tapes, and other things. Some OS' were particularly sensitive about it.
Fun fact, in the early days you could troll someone on certain terminals by having their terminal send a ^Y, which would lock that terminal until it restarted. Fortunately you couldn't hear either the laughing or swearing thanks to the deafening roar from banks of air handlers