what is the 'en' under those XIB files in XCode?

I'm looking at a sample code from Apple; named 'iPhoneCoreDataRecipes' and found those 'en' files under DetailHeaderView.xib, RecipeAddView.xib and InstructionsView.xib.

Wonder what are they and their purpose.

Why just make one DetailHeaderView.xib file without the 'en' file?

please find the capture screen shot from开发者_运维知识库 the link below: http://i.stack.imgur.com/Wulc1.png

It is for localization. 'en' means English. You could have other files for other languages.