经过一些研究，我得出以下结论

// Get the posts in that category with the required tag
$args = array(
    \'post_type\' => [\'documents\',\'news\'],
    \'fields\'          => \'ids\', // Only get post IDs
    \'posts_per_page\'  => -1,
    \'tax_query\'      => array(
        array(
            \'taxonomy\'  => \'forms\',
            \'field\'     => \'slug\',
            \'terms\'     => $symbol
        )
    )
);
$posts_array = get_posts( $args );
$categories = wp_get_object_terms($posts_array, [\'sources\',\'articles\'], [\'fields\' => \'id=>name\']);