Meta Box Flex syntax

<?php

add_filter( 'rwmb_meta_boxes', function( $meta_boxes ) {
    // Flex and flex-item type.
    $meta_boxes[] = array(
        'title' => 'Meta box flex simple',
        'fields' => array(
            array(
                'type' => 'flex', // This will define a flex container.
                'flex' => array(), // Flex attributes.
                'fields' => array(
                    array(
                        'type' => 'flex-item',
                        'flex' => array(), // Flex attributes.
                        'fields' => array(
                            array(
                                'type' => 'text',
                                'id'   => 'field-a',
                                'title' => 'Field A',
                            ),
                            array(
                                'type' => 'text',
                                'id'   => 'field-b',
                                'title' => 'Field B',
                            ),
                        ),
                    ),
                    array(
                        'type' => 'flex-item',
                        'flex' => array(), // Flex attributes.
                        'fields' => array(
                            array(
                                'type' => 'text',
                                'id'   => 'field-c',
                                'title' => 'Field C',
                            ),
                        ),
                    ),
                ),
            ),
        ),
    );

    $meta_boxes[] = array(
        'title' => 'Meta box flex in a group',
        'fields' => array(
            array(
                'type' => 'group',
                'fields' => array(
                    array(
                        'type' => 'flex', // This will define a flex container.
                        'flex' => array(), // Flex attributes.
                        'fields' => array(
                            array(
                                'type' => 'flex-item',
                                'flex' => array(), // Flex attributes.
                                'fields' => array(
                                    array(
                                        'type' => 'text',
                                        'id'   => 'field-a',
                                        'title' => 'Field A',
                                    ),
                                    array(
                                        'type' => 'text',
                                        'id'   => 'field-b',
                                        'title' => 'Field B',
                                    ),
                                ),
                            ),
                            array(
                                'type' => 'flex-item',
                                'flex' => array(), // Flex attributes.
                                'fields' => array(
                                    array(
                                        'type' => 'text',
                                        'id'   => 'field-c',
                                        'title' => 'Field C',
                                    ),
                                ),
                            ),
                        ),
                    ),
                ),
            ),
        ),
    );

    return $meta_boxes;
} );