title: Meaning of the middle line on xcode asset slicing?
body: <p>Official documentaion:</p>
  
  <p><a href="https://help.apple.com/xcode/mac/8.0/#/deve65bd8d0d" rel="noreferrer">https://help.apple.com/xcode/mac/8.0/#/deve65bd8d0d</a></p>
  
  <p>3 vertical lines:
  - left line shows the left unstretchable area
  - right line shows the right unstretchable area
  But can't understand what does the middle vertical line actually do. Could you explain how does it work?</p>