There is only one file type that uses the .USD extension:
Universal Scene Description Format
A USD file contains a 3D scene saved in the Universal Scene Description (USD) format, which is developed by Pixar Animation Studios. It stores information about a 3D scene, which includes 3D geometry, shading data, and references to external assets included in the scene.
The USD format was developed by Pixar to better support their 3D graphics pipeline for their animations. One of the key features of the format is its interchangeability, which allows different 3D applications utilized in the Pixar 3D graphics pipeline to define, package, assemble, and edit the 3D data in the USD format.
Another key feature is its support for collaboration. This involves different "layers" in the format which are separate files that different collaborating artists can non-destructively work on individually and combine together to make up a single scene.
The .usd file extension is a generic extension used by versatile USD files that may be saved in the proprietary "crate" binary format or in an ASCII format. However, USD files saved in the "crate" format typically use the .usdc file extension, and USD files saved in the ASCII format typically use the .usda file extension.
NOTE: The USD format is open source.
Updated: June 7, 2020