http://api.drupal.org/api/drupal/developer!topics!forms_api_reference.ht...
States:
exists: A function name to invoke for checking whether a submitted machine name value already exists. The submitted value is passed as argument.
The full signature for this callback is shown in form.inc where it is called:
if ($function($element['#value'], $element, $form_state)) {
Closed: won't fix
API documentation files
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Thanks for working to improve Drupal.
This is for the Drupal 7 Form API reference. Since Drupal 7 is past end-of-life this is now a "won't fix".
Work on documentation for form elements for supported versions of Drupal is at 📌 [meta] Move/Create Form Element Documentation Active and 🌱 [Meta] Improve documentation for Render and Form Elements Active .