WordPress at a glance
filter is not described

nav_menu_submenu_css_class filter-hook . WP 4.8.0

Filters the CSS class(es) applied to a menu list element.

Usage

add_filter( 'nav_menu_submenu_css_class', 'filter_function_name_2307', 10, 3 );
function filter_function_name_2307( $classes, $args, $depth ){
	// filter...

	return $classes;
}
$classes(array)
The CSS classes that are applied to the menu <ul> element.
$args(stdClass)
An object of wp_nav_menu() arguments.
$depth(int)
Depth of menu item. Used for padding.

Where the hook is called

Walker_Nav_Menu::start_lvl()
wp-includes/class-walker-nav-menu.php 72
$class_names = join( ' ', apply_filters( 'nav_menu_submenu_css_class', $classes, $args, $depth ) );

Where the hook is used (in WP core)

Does not used.