Walker_Nav_Menu::start_lvl( string $output, int $depth = 0, stdClass $args = null )

70	public function start_lvl( &$output, $depth = 0, $args = null ) {71		if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {72			$t = '';73			$n = '';74		} else {75			$t = "\t";76			$n = "\n";77		}78		$indent = str_repeat( $t, $depth );79 80		// Default class.81		$classes = array( 'sub-menu' );82 83		/**84		 * Filters the CSS class(es) applied to a menu list element.85		 *86		 * @since 4.8.087		 *88		 * @param string[] $classes Array of the CSS classes that are applied to the menu `<ul>` element.89		 * @param stdClass $args    An object of `wp_nav_menu()` arguments.90		 * @param int      $depth   Depth of menu item. Used for padding.91		 */92		$class_names = implode( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );93 94		$atts          = array();95		$atts['class'] = ! empty( $class_names ) ? $class_names : '';96 97		/**98		 * Filters the HTML attributes applied to a menu list element.99		 *100		 * @since 6.3.0101		 *102		 * @param array $atts {103		 *     The HTML attributes applied to the `<ul>` element, empty strings are ignored.104		 *105		 *     @type string $class    HTML CSS class attribute.106		 * }107		 * @param stdClass $args      An object of `wp_nav_menu()` arguments.108		 * @param int      $depth     Depth of menu item. Used for padding.109		 */110		$atts       = apply_filters( 'nav_menu_submenu_attributes', $atts, $args, $depth );111		$attributes = $this->build_atts( $atts );112 113		$output .= "{$n}{$indent}<ul{$attributes}>{$n}";114	}