WordPress Code Reference

nav_menu_submenu_css_class

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

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

$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.

Changelog
Version	Description
4.8.0	Introduced.