title: Angular2 Directive - selector with multiple ids
body: <p>I'm implementing a custom directive in Angular 2, directive for form validation, and in many places I see that in the directive definition the selector property is associated with multiple ids - for example:</p>
  
  <pre><code>@Directive({
      selector: '[my-custom-validator][ngModel]'
  })
  </code></pre>
  
  <p>What does the multiple '[...]' (brackets) selection mean?</p>