There is only one file type that uses the .XIB extension:
Interface Builder File
Application interface created with Interface Builder, a graphical editor for designing and testing user interfaces; saved in a text-based flat-file format, introduced in Interface Builder 3.0.
NOTE: XIB files are also called "development-time format" files. Previous versions of Interface Builder saved files in a "deployment format" with a .NIB extension. Interface Builder 3 and later (included with Apple Xcode) can save files in both the XIB and NIB formats.
|Apple Interface Builder|
Updated: August 19, 2008