title: Xcode adding objective-c file "file type"
body: <p>I'm trying to add a new file to my Xcode project using Xcode 6.1.1 and Xcode now has a "File type" option where you select between "Empty File, Category, Protocol, Extension"</p>
  
  <p>Can someone explain the differences between these and what the default to select would be? My file is a subclass of NSObject.</p>
  
  <p>Thanks</p>