Examples
Some examples from extension styleguide to get an idea on what the field definition is capable of: An input field with slider, a select drop-down for images, an inline relation spanning multiple tables.
The following examples all can be found in the extension styleguide.
Select drop-down for records represented by images
 
            
            Select field with foreign table relation and field wizard:
[
    'columns' => [
        'select_single_12' => [
            'label' => 'select_single_12 foreign_table selicon_field',
            'config' => [
                'type' => 'select',
                'renderType' => 'selectSingle',
                'foreign_table' => 'tx_styleguide_elements_select_single_12_foreign',
                'fieldWizard' => [
                    'selectIcons' => [
                        'disabled' => false,
                    ],
                ],
            ],
        ],
    ],
]The table 
        tx_ is defined as
follows:
[
   'ctrl' => [
      'title' => 'Form engine elements - select foreign single_12',
      'label' => 'fal_1',
      'selicon_field' => 'fal_1',
      // ...
   ],
   'columns' => [
      // ...
      'fal_1' => [
         'label' => 'fal_1 selicon_field',
         'config' => \TYPO3\CMS\Core\Utility\ExtensionManagementUtility::getFileFieldTCAConfig(
            'fal_1',
            [
               'maxitems' => 1,
            ],
            $GLOBALS['TYPO3_CONF_VARS']['SYS']['mediafile_ext']
         ),
      ],
   ],
   // ...
];Inline relation (IRRE) spanning multiple tables
 
            
            Inline relation to a foreign table:
[
    'columns' => [
        'inline_1' => [
            'label' => 'inline_1',
            'config' => [
                'type' => 'inline',
                'foreign_table' => 'tx_styleguide_inline_1n1n_child',
                'foreign_field' => 'parentid',
                'foreign_table_field' => 'parenttable',
            ],
        ],
    ],
]Example: prefixLangTitle
The following example can be found in the extension styleguide. On translating a record in a new language the content of the
field gets copied to the target language. It get prefixed with
[Translate to <language name>:].
 
            
            The language mode is defined as follows:
[
    'columns' => [
        'text_2' => [
            'l10n_mode' => 'prefixLangTitle',
            'label' => 'text_2',
            'description' => 'cols=20',
            'config' => [
                'type' => 'text',
                'cols' => 20,
            ],
        ],
    ],
]Disable the prefixLangTitle for the header field in tt_content
Use the default behaviour instead of 
        prefix: the field will
be copied without a prepended string.
$GLOBALS['TCA']['tt_content']['columns']['header']['l10n_mode'] = ''Select field with defaultAsReadonly  
            
    The following field has the option 
        'l10n_
set:
 
            
            Complete TCA definition of the field:
[
    'columns' => [
        'select_single_13' => [
            'label' => 'select_single_13 l10n_display=defaultAsReadonly',
            'l10n_mode' => 'exclude',
            'l10n_display' => 'defaultAsReadonly',
            'config' => [
                'type' => 'select',
                'renderType' => 'selectSingle',
                'items' => [
                    [
                        'label' => 'foo',
                        'value' => 'foo',
                    ],
                    [
                        'label' => 'bar',
                        'value' => 'bar',
                    ],
                ],
            ],
        ],
    ],
]Translated field without l10n_display  definition
            
    The following has no 
        'l10n_ definition:
 
            
            Complete TCA definition of the field:
[
    'columns' => [
        'select_single_8' => [
            'label' => 'select_single_8 drop down with empty div',
            'config' => [
                'type' => 'select',
                'renderType' => 'selectSingle',
                'items' => [
                    [
                        'label' => 'First div with items',
                        'value' => '--div--',
                    ],
                    [
                        'label' => 'item 1',
                        'value' => 1,
                    ],
                    [
                        'label' => 'item 2',
                        'value' => 2,
                    ],
                    [
                        'label' => 'Second div without items',
                        'value' => '--div--',
                    ],
                    [
                        'label' => 'Third div with items',
                        'value' => '--div--',
                    ],
                    [
                        'label' => 'item 3',
                        'value' => 3,
                    ],
                ],
            ],
        ],
    ],
]