$this->fields_options = array(
        'general' => array(
            ['title'] => $this->l('Carrier options'),                          // if missing, default 'Options'
            ['top'] => $this->l('Text to display before the fieldset'),
            ['image'] => 'url to icon'                                         // if missing will use the default icon for the tab
            ['description'] => $this->l('Display as description'),
            ['info'] => $this->l('Display as info'),
            'fields' => array(
                'PS_CARRIER_DEFAULT' => array(
                    ['title'] => $this->l('Default carrier:'),
                    ['desc'] => $this->l('The default carrier used in shop'),
                    ['cast'] => 'intval',
                    'type' => {'text', 'hidden', 'select', 'bool', 'radio', 'checkbox', 'password', 'textarea', 'file', 'textLang', 'textareaLang', 'selectLang'},
                    ['suffix'] => 'Display after the field (ie. currency),     // only for text or password
                    'identifier' => 'id_carrier',
                    ['list'] => array(list do display as options),             // only for select
                    ['empty_message'] => $this->l('Display if list is empty'), // only for select
                    ['cols'] => 40,                                            // for textarea
                    ['rows'] => 5,                                             // for textarea
                    ['thumb'] => 'url to thumb image'                          // for files
                    ),
                    ['is_invisible'] => {true, false}                          // Disable the field depending on shop context
                ),
                'ANOTHER_FIELD' => ...
            ),
            'submit' => array()
        ),
        'another fieldset' => ...
    );