apply_filters( 'nav_menu_submenu_css_class', string[] $classes, stdClass $args, int $depth )

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

Parameters

$classes: Array of the CSS classes that are applied to the menu <ul> element.
$args: An object of wp_nav_menu() arguments.
$depth: Depth of menu item. Used for padding.

Source

File: wp-includes/class-walker-nav-menu.php

Changelog

Changelog
Version	Description
4.8.0	Introduced.