Holds temporary data accessible during the current page request only.
All $form_state properties that are not reserved keys (see other properties marked as uncacheable) persist throughout a multistep form sequence. Form API provides this key for modules to communicate information across form-related functions during a single page request. It may be used to temporarily save data that does not need to or should not be cached during the whole form workflow; e.g., data that needs to be accessed during the current form build process only. There is no use-case for this functionality in Drupal core.
This property is uncacheable.
Type: array
protected $temporary = [];
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Form!FormState.php/property/FormState::temporary/8.1.x