Hierarchical select/checkbox_list

demo.php

<?php
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
	$meta_boxes[] = array(
		'title'  => 'Test',
		'fields' => array(
			array(
				'type'    => 'select_tree', // or checkbox_list
				'id'      => 'cb',
				'name'    => 'Hierarchical checkboxes',
				'options' => array(
					array( 'value' => 'value1', 'label' => 'label 1' ),
					array( 'value' => 'value2', 'label' => 'label 2' ),
					array( 'value' => 'sub1', 'label' => 'Sub 1', 'parent' => 'value1' ),
					array( 'value' => 'sub2', 'label' => 'Sub 2', 'parent' => 'value1' ),
				),
				'flatten' => false,
			),
		),
	);

	return $meta_boxes;
} );