UASSET is an exclusive format to Unreal Engine 4 games. It can be anything beside maps, such as Models, Materials, Textures, Blueprints, etc. They cannot be opened on Unreal Engine 4 directly because they are "cooked". They cannot be renamed for simple swap as the game will either crash or softlock. However, they can be made, cooked, and replaced directly with the usage of Unreal Engine 4.