It matters because if you answered something other than RAd/RSA 6/7/8, you'd have a problem. Believe it or not, some people are still using WSAD 5! Or you could have been using pure Eclipse.
The system library only gets added automatically for certain types of projects. Try adding a new web or ejb project and you'll see it. For a pure Java project, you can still add the library though. You go to the build path for the project and choose add library. You'll see the server library in the list of choices.