fieldInformation 

The fieldInformation is a reserved area within a single form element between the label and the form element itself.

fieldInformation
Type
array
TCA path
$GLOBALS['TCA'][$table]['columns'][$field]['config']['fieldInformation']
Scope
Display

Currently, TYPO3 comes with following implemented fieldInformation nodes:

Example 

The field tags shows a translated information text next to its label. The label reference of the text is passed to the node in the options of its fieldInformation configuration:

EXT:my_extension/Configuration/TCA/tx_myextension_product.php 
<?php

return [
  'ctrl' => [
    'title' => 'Product',
    'label' => 'tags',
  ],
  'columns' => [
    'tags' => [
      'label' => 'Tags',
      'config' => [
        'type' => 'input',
        'fieldInformation' => [
          'tagInformation' => [
            'renderType' => 'myExtensionTagInformation',
            'options' => [
              'text' => 'my_extension.db:tags.information',
            ],
          ],
        ],
      ],
    ],
  ],
  'types' => [
    '0' => [
      'showitem' => 'tags',
    ],
  ],
];
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The node receives the options in $this->data['renderData']['fieldInformationOptions'] and returns the HTML to show:

EXT:my_extension/Classes/Backend/FieldInformation/TagInformation.php 
<?php

declare(strict_types=1);

namespace MyVendor\MyExtension\Backend\FieldInformation;

use TYPO3\CMS\Backend\Form\AbstractNode;

final class TagInformation extends AbstractNode
{
  public function render(): array
  {
    $result = $this->initializeResultArray();
    $label = $this->data['renderData']['fieldInformationOptions']['text'];
    $result['html'] = htmlspecialchars(
      $GLOBALS['LANG']->sL($label),
    );
    return $result;
  }
}
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The node is registered with the name used as renderType in ext_localconf.php:

EXT:my_extension/ext_localconf.php 
<?php

use MyVendor\MyExtension\Backend\FieldInformation\TagInformation;

defined('TYPO3') or die();

$GLOBALS['TYPO3_CONF_VARS']['SYS']['formEngine']['nodeRegistry'][1552726986] = [
  'nodeName' => 'myExtensionTagInformation',
  'priority' => 40,
  'class' => TagInformation::class,
];
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