title: Apple Headers contain multiple Categories
body: <p>While perusing some Apple Header files, I notice that they declare multiple interfaces using Categories for the same object.</p>
  
  <p>For example: <code>NSDictionary.h</code></p>
  
  <pre><code>@interface NSDictionary : NSObject &lt;NSCopying, NSMutableCopying, NSCoding, NSFastEnumeration&gt;
  
  //methods
  
  @end
  
  @interface NSDictionary (NSExtendedDictionary)
  
  //methods
  
  @end
  
  @interface NSDictionary (NSDictionaryCreation)
  
  //methods
  
  @end
  </code></pre>
  
  <p>Is this purely a way to help organize the code? As a user there is no difference, all methods in all Categories appear when utilizing a <code>NSDictionary</code>. </p>
  
  <p>Is there some other useful result of this?</p>