title: What does "Constrain to margins" mean in Interface Builder in Xcode 6.0.1
body: <p>I understand the purpose of "Spacing to the nearest neighbor" (in Xcode 6, more especially in Interface Builder) but <strong>what is the point of "Constrain to margins"?</strong></p>
  
  <p><strong>Is there a link between "Spacing to the nearest neighbor" an edge of a particular control button, and the fact of at the same time checking the checkbox named "Constrain to margins"?</strong></p>
  
  <p><strong>What is the role of this feature in Autolayout?</strong></p>
  
  <p>Please find a screenshot with the checkbox and the label that I am talking about surrounded in red below:</p>
  
  <p><img src="https://i.stack.imgur.com/5SVBL.png" alt="Constrain to margins screenshot"></p>
  
  <p>(<a href="https://i.stack.imgur.com/5SVBL.png" rel="noreferrer">Screenshot above in bigger format</a>)</p>
  
  <p>Thank you in advance for your answers to my three questions.</p>