TRUE signifies correct form submission. This is always TRUE for programmed forms coming from \Drupal\Core\Form\FormBuilderInterface::submitForm() (see 'programmed' key), or if the form_id coming from the \Drupal::request()->request data is set and matches the current form_id.
Type: bool
protected $process_input;
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Form!FormState.php/property/FormState::process_input/8.1.x