What is a USX file?
A USX file contains Unreal Engine’s meshes that are pre-designed 3D objects/models used to generate the environment. Models from USX file have different position, orientation and sizes, but these remain static. Static Meshes can be optimized for performance and can be used to create various levels of detail (LODs) to improve the game’s performance. They can also be used to create collision volumes and can be used as part of physics simulations.
USX files can be created with Unreal level editor “UnrealEd”.
USX File Format - More Information
USX files are saved to disc as binary files. The internal file structure details of USX file format are not available publicly for developer’s reference.