The X file format refers to files with the .x file name extension. X files were introduced with DirectX 2.0. A binary version of this format was subsequently released with DirectX 3.0, which is also described in this documentation. DirectX
6.0 introduced interfaces and methods that enable reading from and writing to .x files. Support was maintained through Direct3D 9, but was deprecated for Direct3D 10 and later.
The XFile APIs are only supported by the legacy DirectX SDK in the D3DX9 DLL.
The binary version of X files were intended as a runtime format, but the text version of X files have become a source file used by various community model libraries, and is supported by various freeware tools.
File Format Details