title: What does XCode 4.2 story builder's "Defines Context" and "Provide Context" mean?
body: <p>In the Navigation Controller Attributes Inspector under View Controller section there are
  the "Defines Context" and "Provide Context" check boxes.</p>
  
  <p><img src="https://i.stack.imgur.com/Zfq7s.png" alt="enter image description here"></p>
  
  <p>I cant find any documentation on this features.
  How to use it and what does it do?</p>