Ok, I will explain it.
The files you download from NUS are NUS files that can be installed directly to your console, provided you have a ticket file.
A ticket file can be pulled from the disc, or fakesigned if we only have the title key, but then you need sig patches to run the game.
A WUD is a Wii U Disc Image, which is an exact copy of the disc, which can be decrypted with the title key and made into Loadiine ready files or NUS files for the USB method.
The NUS files can also be decrypted into Loadiine ready files, but Loadiine is a pile of trash and nobody should use it anymore.