Normally, after the entire form processing is completed and submit handlers have run, a form is considered to be done and \Drupal\Core\Form\FormSubmitterInterface::redirectForm() will redirect the user to a new page using a GET request (so a browser refresh does not re-submit the form). However, if 'rebuild' has been set to TRUE, then a new copy of the form is immediately built and sent to the browser, instead of a redirect. This is used for multi-step forms, such as wizards and confirmation fo