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:

EXT:styleguide/Configuration/TCA/tx_styleguide_elements_select.php 
[
    '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,
                    ],
                ],
            ],
        ],
    ],
]
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The table tx_styleguide_elements_select_single_12_foreign is defined as follows:

EXT:styleguide/Configuration/TCA/tx_styleguide_elements_select_single_12_foreign.php 
<?php

return [
  'ctrl' => [
    'title' => 'Form engine elements - select foreign single_12',
    'label' => 'fal_1',
    'iconfile' => 'EXT:styleguide/Resources/Public/Icons/tx_styleguide.svg',
    'tstamp' => 'tstamp',
    'crdate' => 'crdate',
    'delete' => 'deleted',
    'sortby' => 'sorting',
    'selicon_field' => 'fal_1',
    'languageField' => 'sys_language_uid',
    'transOrigPointerField' => 'l10n_parent',
    'transOrigDiffSourceField' => 'l10n_diffsource',
    'translationSource' => 'l10n_source',
    'enablecolumns' => [
      'disabled' => 'hidden',
    ],
    'security' => [
      'ignorePageTypeRestriction' => true,
    ],
  ],

  'columns' => [
    'fal_1' => [
      'label' => 'fal_1 selicon_field',
      'config' => [
        'type' => 'file',
        'allowed' => 'common-media-types',
        'relationship' => 'manyToOne',
      ],
    ],
  ],

  'types' => [
    '0' => [
      'showitem' => 'fal_1',
    ],
  ],

];
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Inline relation (IRRE) spanning multiple tables 

Inline relation to a foreign table:

EXT:styleguide/Configuration/TCA/tx_styleguide_inline_1n1n.php 
[
    'columns' => [
        'inline_1' => [
            'label' => 'inline_1',
            'config' => [
                'type' => 'inline',
                'foreign_table' => 'tx_styleguide_inline_1n1n_child',
                'foreign_field' => 'parentid',
                'foreign_table_field' => 'parenttable',
            ],
        ],
    ],
]
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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:

EXT:styleguide/Configuration/TCA/tx_styleguide_elements_basic.php 
[
    'columns' => [
        'text_2' => [
            'l10n_mode' => 'prefixLangTitle',
            'label' => 'text_2',
            'description' => 'cols=20',
            'config' => [
                'type' => 'text',
                'cols' => 20,
            ],
        ],
    ],
]
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Disable the prefixLangTitle for the header field in tt_content 

Use the default behaviour instead of prefixLangTitle: the field will be copied without a prepended string.

EXT:my_sitepackage/Configuration/TCA/Overrides/tt_content.php 
$GLOBALS['TCA']['tt_content']['columns']['header']['l10n_mode'] = '';
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Select field with defaultAsReadonly 

The following field has the option 'l10n_display' => 'defaultAsReadonly' set:

Complete TCA definition of the field:

EXT:styleguide/Configuration/TCA/tx_styleguide_elements_select.php 
[
    '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',
                    ],
                ],
            ],
        ],
    ],
]
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Translated field without l10n_display definition 

The following has no l10n_display definition:

Complete TCA definition of the field:

EXT:styleguide/Configuration/TCA/tx_styleguide_elements_select.php 
[
    '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,
                    ],
                ],
            ],
        ],
    ],
]
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