the use of role attribute

Hi guys,

I am studying bootstrap from W3schools.
what is the use of 'role' attribute?
can it have a different value than 'menu' here?
Is it required?

<div class="btn-group">
  <button type="button" class="btn btn-primary">Apple</button>
  <button type="button" class="btn btn-primary">Samsung</button>
  <div class="btn-group">
    <button type="button" class="btn btn-primary dropdown-toggle" data-toggle="dropdown">
    Sony <span class="caret"></span></button>
    <ul class="dropdown-menu" [b]role="menu"[/b]>
      <li><a href="#">Tablet</a></li>
      <li><a href="#">Smartphone</a></li>
    </ul>
  </div>
</div>

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