There is only one file type that uses the .VHDX extension:
Windows 8 Virtual Hard Drive File
Disk image supported by the Windows 8 and Windows Server 8 operating systems; extends the .VHD format with new capabilities, such as performance improvements, data corruption prevention mechanisms, and a 16TB maximum allowable size.
Microsoft Windows Server 8 includes Microsoft Hyper-V technology, which can use disk images to bootstrap virtual machines.
Updated: February 23, 2019