Just guessing but I think the closed end lug nut is like an Acorn nut,,,it has a top to it so you cannot see the threaded stud your lug nut goes on . An open lug nut with no top you will see the wheel stud after you have threaded the lug nut on,,

 

So for aftermarket wheels which is better?

 

You would want the closed nut for better looks fur sure,(if my guess above is correct,,,,)