Bootstrap表单控件，bs-example bs-example-form是有什么用途的？

   <form class="bs-example bs-example-form" role="form">
        <div class="input-group input-group-lg">
            <span class="input-group-addon">@</span>
            <input type="text" class="form-control" placeholder="Twitterhandle">
        </div>