Optional option on forms and settings
const DRUPAL_OPTIONAL = 1;
© 2001–2016 by the original authors
Licensed under the GNU General Public License, version 2 and later.
Drupal is a registered trademark of Dries Buytaert.
https://api.drupal.org/api/drupal/core!modules!system!system.module/constant/DRUPAL_OPTIONAL/8.1.x