bbPress::register_taxonomies()

    public static function register_taxonomies() {

        // Register the topic-tag taxonomy
        register_taxonomy(
            bbp_get_topic_tag_tax_id(),
            bbp_get_topic_post_type(),
            apply_filters( 'bbp_register_topic_taxonomy', array(
                'labels'                => bbp_get_topic_tag_tax_labels(),
                'rewrite'               => bbp_get_topic_tag_tax_rewrite(),
                'capabilities'          => bbp_get_topic_tag_caps(),
                'update_count_callback' => '_update_post_term_count',
                'query_var'             => true,
                'show_tagcloud'         => true,
                'hierarchical'          => false,
                'show_in_nav_menus'     => false,
                'public'                => true,
                'show_ui'               => bbp_allow_topic_tags() && current_user_can( 'bbp_topic_tags_admin' ),
                'show_in_menu'          => false,
            )
        ) );
    }