title: What precisely is gamut in size classes?
body: <p>When you want to add a variation to an attribute, you get this popup (I'm using Xcode 8) :</p>
  
  <p><a href="https://i.stack.imgur.com/avPoe.png" rel="noreferrer"><img src="https://i.stack.imgur.com/avPoe.png" alt="enter image description here"></a></p>
  
  <p>As far as I understand, gamut setting here is for a display type. But I don't understand what it really implies and when I should use it ?</p>