How do I use the Spacing Utility Classes in Bootstrap

In this article I saw Bootstrap 4 Spacing Utility Classes, and he uses m-b-lg in className.

<div class="row">
    <div class="col-sm-6 m-b-lg">
        <!-- column-small-50%, margin-bottom-large -->
    </div>
</div>

But when I use it on meteorjs with react nothing happens. Am I missing something or missing a plugin?

<div className="container-fluid">
    <div className="col-xs-6 col-xs-offset-3 m-t-lg">
        <h1 className="text-xs-center"> Login </h1>
        <form>
            <div className="form-group">
                <input type="email" className="form-control" id="inputEmail" placeholder="Email"/>
            </div>
            <div className="form-group">
                <input type="password" className="form-control" id="inputPassword" placeholder="Password"/>
                <p className="text-xs-center"><a href="/signup"> Forgot your email or password?</a></p>
            </div>      
            <div className="form-group">
                <button className="btn btn-primary btn-block" type="submit"> Login </button>
                <p className="text-xs-center"> New to Arcademy? <a href="/signup"> Sign up now.</a></p>
            </div>
        </form>
    </div>
</div>

Answer

Refer to the Spacing (Bootstrap v4 alpha) documentation.

The classes are named using the format: {property}-{sides}-{size}

Where size is one of: * 0 – for classes that eliminate the margin or padding by setting it to 0 * 1 – (by default) for classes that set the margin or padding to $spacer-xor $spacer-y * 2 – (by default) for classes that set the margin or padding to $spacer-x * 1.5 or $spacer-y * 1.5 * 3 – (by default) for classes that set the margin or padding to $spacer-x * 3 or $spacer-y * 3.

So use m-t-3 instead of m-t-lg.