WordPress at a glance
filter is not described

wp_setup_nav_menu_item filter-hook . WP 3.0.0

Filters a navigation menu item object.

Usage

add_filter( 'wp_setup_nav_menu_item', 'filter_function_name_5412' );
function filter_function_name_5412( $menu_item ){
	// filter...

	return $menu_item;
}
$menu_item(object)
The menu item object.

Where the hook is called

wp_setup_nav_menu_item()
WP_Customize_Nav_Menu_Item_Setting::value_as_wp_post_nav_menu_item()
wp-includes/nav-menu.php 906
return apply_filters( 'wp_setup_nav_menu_item', $menu_item );
wp-includes/customize/class-wp-customize-nav-menu-item-setting.php 630
$post = apply_filters( 'wp_setup_nav_menu_item', $post );

Where the hook is used (in WP core)

Does not used.