What does #optional mean for a "details" form element?

The node edit form contains the following form element.

  $form['revision_information'] = array(     '#type' => 'details',     '#group' => 'advanced',     '#title' => t('Revision information'),     // Open by default when "Create new revision" is checked.     '#open' => $node->isNewRevision(),     '#attributes' => array(       'class' => array('node-form-revision-information'),     ),     '#attached' => array(       'library' => array('node/drupal.node'),     ),     '#weight' => 20,     '#optional' => TRUE,   );

What does the #optional property mean for it?

I looked at both the Details and the VerticalTabs classes, but neither of them seems to handle that property.

What does setting #optional to TRUE do?

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What does #optional mean for a "details" form element?

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