When Windows 10 build 1903 is released, it will introduce a new feature called Reserved Storage. Microsoft aims to make Windows 10 more reliable by reserving enough storage space so that updates can be installed even if the system drive has no more free space. Microsoft says the amount of reserved storage space could increase over time as temporary files created by apps and system will automatically be placed into reserved storage. Reserved Storage will be available only on devices with Windows 10 19H1 (version 1903) pre-installed or those where 1903 was clean installed.
Source: https://www.bleepingcomputer.com/news/microsoft/how-reserved-storage-works-in-the-next-version-of-windows-10/