title: When should I use "ngProjectAs" attribute?
body: <p>I've come across the following code where <code>ngProjectAs</code> attribute is being used within <code>ng-container</code>. what is it used for, what problem does it address?</p>
  <pre class="lang-html prettyprint-override"><code>&lt;mat-form-field [formGroup]=&quot;group&quot;&gt;
    &lt;input matInput [formControlName]=&quot;field.name&quot;&gt;
  
    &lt;ng-container *ngFor=&quot;let validation of field.validations;&quot; ngProjectAs=&quot;mat-error&quot;&gt;
      &lt;mat-error *ngIf=&quot;group.get(field.name).hasError(validation.name)&quot;&gt;{{validation.message}}&lt;/mat-error&gt;
    &lt;/ng-container&gt;
  &lt;/mat-form-field&gt;
  </code></pre>