Diffrence between Xib and Nib

As of Interface Builder version 3, a new file format (with extension .xib) has been added, which is functionally identical to .nib, except it is stored in a flat file, making it more suitable for storage in revision control systems and processing by tools such as diff.

The XIB is compiled into a NIB file for inclusion in your app.

The compiled NIB that is included in the app bundle is no longer editable in Interface Builder and is much smaller than the equivalent XIB or legacy NIB file.