Part 1 of this 2-part series covers the details of how the Universal Scene Description (USD) format can be structured to author and import Nanite Assemblies, a new Unreal Engine 5.7 feature enabling micro-instancing of geometry within a single mesh asset to optimize it for memory usage.
https://dev.epicgames.com/community/learning/tutorials/7O3z/unreal-engine-nanite-assemblies-in-usd-part-1-scene-structure