Twitter Bootstrap 4 navbar-toggleable-*

Given the code below is the supposed behavior that the button is hidden at lg and xl; while the collapsed content is only toggeable at md or is it toggeable from md downto xs? Can someone help explain this?

<nav class="navbar navbar-light bg-faded">
  <button class="navbar-toggler hidden-lg-up" type="button" data-toggle="collapse" data-target="#exCollapsingNavbar2">
    &#9776;
  </button>
  <div class="collapse navbar-toggleable-md" id="exCollapsingNavbar2">
    <a class="navbar-brand" href="#">Responsive navbar</a>
    <ul class="nav navbar-nav">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Features</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Pricing</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">About</a>
      </li>
    </ul>
  </div>
</nav>