Microsoft is testing Insider builds that use the modern ReFS file format instead of the more common NTFS for a clean installation of Windows 11. When you install a supported version of Windows using an ISO or from a USB drive, you can choose ReFS or the default NTFS format when you reach the “setup” screen, where you enter the region and language and your license key. While not explicitly mentioned in Microsoft’s release notes for Insider build 27823 (Canary), PhantomofEarth on X noticed this Flexible Storage option.