https://www.bleepingcomputer.com/news/microsoft/windows-10-build-18312-19h1-released-with-reserved-storage-implementation/

Windows 10 Preview Build 18312 is now available for Insiders in the Fast Ring and it comes with Reserved Storage implementation, improved Reset page UI, Windows Subsystem for Linux Command Line Tool improvements. Overall, though, this build is more about general improvements and fixes as Microsoft appears to be focusing more on stabilization of the OS.

In Windows 10 19H1 Build 18312, you'll notice a new Reserved Storage if you open the Storage settings, click “Show more categories” and access the System & reserved page. Microsoft says that implementation of Reserved Storage in Windows 10 is supposed to improve the day-to-day function of your PC by ensuring critical OS functions always have access to disk space.