Source

File: wp-includes/taxonomy.php

function register_taxonomy_for_object_type( $taxonomy, $object_type ) {
	global $wp_taxonomies;

	if ( ! isset( $wp_taxonomies[ $taxonomy ] ) ) {
		return false;
	}

	if ( ! get_post_type_object( $object_type ) ) {
		return false;
	}

	if ( ! in_array( $object_type, $wp_taxonomies[ $taxonomy ]->object_type, true ) ) {
		$wp_taxonomies[ $taxonomy ]->object_type[] = $object_type;
	}

	// Filter out empties.
	$wp_taxonomies[ $taxonomy ]->object_type = array_filter( $wp_taxonomies[ $taxonomy ]->object_type );

	/**
	 * Fires after a taxonomy is registered for an object type.
	 *
	 * @since 5.1.0
	 *
	 * @param string $taxonomy    Taxonomy name.
	 * @param string $object_type Name of the object type.
	 */
	do_action( 'registered_taxonomy_for_object_type', $taxonomy, $object_type );

	return true;
}