WordPress at a glance
action is not described

admin_xml_ns action-hook . WP 2.2.0

Fires inside the HTML tag in the admin header.

Usage

add_action( 'admin_xml_ns', 'action_function_name_5988' );
function action_function_name_5988(){
	// action...
}

Where the hook is called

_wp_admin_html_begin()
admin_xml_ns
wp-admin/includes/template.php 2345
do_action( 'admin_xml_ns' );
wp-admin/includes/template.php 2355
do_action( 'admin_xml_ns' );

Where the hook is used (in WP core)

Does not used.