Difference between quarter panel skins vs. full?

Besides the obvious shape difference with the skin not going all the way up to the factory joint with the roof, what is the difference between a replacement quarter panel skin and a full quarter panel?

Is it that the installation is different? Do you actually place a skin directly over an old quarter panel without removing the original?

That seems fishy to me. But other wise it seems to me that you you'd be cutting out old metal to replace either....

I have rust around the rear wheel opening and along the bottom of one of my quarter panels and am trying to determine the best item to purchase for repairing it.

Thanks,
Mike