Currently, avatars will only be registered as fallback avatars if they meet the fallback avatar criteria by enabling the checkbox when uploading them. The standard for fallback avatars is that the avatar performance rank is Good or higher, so why do you need to go to the trouble of uploading them as fallback avatars? If the avatar meets the criteria, we expect a UI that will be displayed at the bottom of the "Change Avatar" button on the VRChat settings screen.
This feature is very useful if you are implementing a tool that performs optimization at build time, and will expand your fallback avatar options.