There is only one file type that uses the .USA extension:
Unreal Saved Game File
Game saved in Unreal or Unreal 2 first-person shooter games; saves the progress of the player and allows the game to be loaded from the point when the game was saved.
Unreal allows up to nine games to be saved at one time. The save files are named "save0.usa" through "save8.usa" and are saved in the "Unreal\Save\" folder. Quick saves are stored in "save9.usa." References to the save files are stored in the "User.ini" file in the same directory.
Updated: December 14, 2008