title: Differences between bootstrap-flex, bootstrap-grid and bootstrap-reboot
body: <p>In the newest version of Bootstrap (current version at the time of asking the question is <code>bootstrap@4.0.0-alpha.5</code>) there are 3 different files and a standard one:</p>
  
  <ol>
  <li>bootstrap-flex.css</li>
  <li>bootstrap-grid.css</li>
  <li>bootstrap-reboot.css</li>
  <li>bootstrap.css</li>
  </ol>
  
  <p>Are the first 3 files an extension to <code>bootstrap.css</code> (#4)?</p>
  
  <p>I can imagine, that the <code>bootstrap-flex</code> use the features of CSS Flexbox, <code>bootstrap-grid</code> is probably the old and already existing way, but what about <code>bootstrap-reboot</code>?</p>
  
  <p>Which one do I have to include (especially then, if I use <a href="https://ng-bootstrap.github.io/#/getting-started" rel="noreferrer">ng-bootstrap</a> with Angular 2)?</p>