.U File Extension

Developers and modders use tools provided by Epic Games, such as ucc.exe (Unreal Command Console), to compile their custom scripts into U files. Once compiled, users can distribute U files and drop them into the appropriate folder to modify or extend a game, similar to a plugin. For example, a modder might create a new weapon or character class in UnrealScript, compile it into a U file, and then load it into Unreal Tournament to use it during gameplay.

U files are typically stored in the game's System folder (for example, \Unreal\System\).  When a game launches, the engine loads the relevant U files from the System directory to access the necessary gameplay code.

U files are not meant to be opened. Instead, the game packaged with the U file references it to load the information it stores.

However, gamers who enjoy modifying gameplay may extract and modify U files with tools, such as Dragon UnPACKer and Unreal Tournament Package Tool.