TYPO3 Core

Example of a Google Map user-defined field in TCA:

'map' => array(
    'label' => 'Google Map',
    'config' => array(
        'type' => 'user',
        'userFunc' => 'Tx_MyExt_Tca_Map->render',
        'parameters' => array(
            'latitude' => 'gps_latitude',
            'longitude' => 'gps_longitude',
        ),
    ),
),

This allows use of "parameters" to provide additional information to the userFunc (here providing the fields used to store latitude/longitude to allow the userFunc to provide a Google Map that dynamically update the coordinates when moving a marker on the map)