what on earth does '#flag_link_fields' do?

  $form['display']['link_type'] = array(
    '#type' => 'radios',
    '#title' => t('Link type'),
    '#options' => _flag_link_type_options(),
    '#option_descriptions' => _flag_link_type_descriptions(),
    '#flag_link_fields' => _flag_link_type_fields(),
    '#after_build' => array('flag_expand_link_option', 'flag_check_link_types'),
    '#default_value' => $flag->link_type,
    '#weight' => 2,
    '#access' => empty($flag->locked['link_type']),
  );

What is '#flag_link_fields'? It doesn't seem to get used anywhere further along the form process.