ImageX is the command-line tool used to create, edit and deploy Windows disk images in the Windows Imaging Format. It is distributed as part of the free Windows Automated Installation Kit (WAIK). Starting with Windows Vista, Windows Setup uses the WAIK API to install Windows.
The first developed prototype of ImageX was build 6.0.4007.0 (main.030212-2037). It allowed Microsoft OEM partners to experiment with the imaging technology and was developed in parallel with Longhorn alpha prototypes. It was first introduced in Milestone 4 into the Longhorn project, and used in later builds of Longhorn. Build 6.0.5384.4 added significant advantages over previous versions, like read-only and read/write folder mounting capabilities, splitting to multiple image files (SWM), a WIM filter driver and the latest LZX compression algorithms. It has been used since pre-RC (release candidates) of Windows Vista.
As of Windows 8, ImageX is replaced by DISM.
Read more about this topic: Windows Imaging Format