meta box attribute to use with gallery

The dev who built this gallery provides the following to output ACF custom fields to define the content of the galleries.
The plugin extends the normal WordPress gallery.

What should I use as an attribute ($atts) to do this with Metabox custom fields?

add_filter( 'shortcode_atts_gallery', function( $result, $defaults, $atts ) {
  if ( !empty( $atts['acf'] ) ) {
    $gallery_id_array = get_field( $atts['acf'], get_the_ID() );
    $ids = implode( ',', $gallery_id_array );
    $result['ids'] = $ids;
  }
  return $result;
}, 10, 3 );