I would say serial number, probably just to make sure that it is yours and not someone else's. I don't know about giving out your product key, as it is propiarty to your system, but I would think the serial number would be enough.
If your paper is about wireless it may be the following:
The security key code is the code thats pops up whenever you finish setting up your wireless adaptor. So no one can use your internet without it..
What is your paper specifically on?