A "fence" extends both up and down, as you can see here:
I assume that the downward extensions also play an important role, since modern aircraft with wing-mounted engines do not have fences, but pylons that do what the fences would do. Again, I'm certainly not an expert at aerodynamics.
By the way, I've always been wondering if this is an exotically-shaped winglet or a wing tip fence:
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Photo © Markus Wisler
EDIT: reading the question once more, I just noticed that I might be wrong. However, my guess is that a winglet curves up (only), but a wing tip fence curves up and down, while it doesn't matter if it covers the entire length of the wing tip.
[Edited 2003-08-30 23:50:04]
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