There are 2 file types use the .U file extension:
1. Unreal Tournament Class Package
Contains settings used by Unreal Tournament; stored within the Unreal System directory, i.e. "\Unreal\System\Engine.u;" may be replaced with custom class packages.
Updated: July 27, 2012
2. AU Audio File
Audio container format typically used to store ?-law encoded audio data; introduced by Sun Microsystems and also used by MusicKit for the NeXT operating system; contains a header that includes six 32-bit words, an optional information section, then the audio data.
AU files typically use the .AU extension.
Updated: July 6, 2014