What is abcd?

A young executive is working late one evening. As he comes out of his office about 8 PM he sees the Big Boss standing by the shredder in the hallway, a piece of paper in his hand. "Do you know how to work this thing?" the older man asks. "My secretary’s gone home and I don’t know how to run it."
"Yes, sir," says the young executive, who turns on the machine, takes the paper from the other man, and feeds it in.
"Now," says his boss, "I just need the one copy."