I’ve done this with get_post_type():
<?php
$rel_field_ids = get_field( 'rel_field' );
$rel_field_ids_grouped = array();
foreach ( $rel_field_ids as $rel_field_id )
{
$post_type = get_post_type( $rel_field_id );
$rel_field_ids_grouped[ $post_type ][] = $rel_field_id;
}
ksort( $rel_field_ids_grouped );
print_r( $rel_field_ids_grouped );
?>
Code not tested.