This value is used by the cgkedit plugin and is defined as:
$_SESSION['dwfck_client'] = $INFO['client'];
So for use outside of ckgedit, you should use:
global $INFO;
$client = $INFO['client'];
Where there is no logged-in user, this will give you the IP address of the machine accessing your page. You can also $_SERVER['REMOTE_USER'], which will either give you a logged-in user or resolve to false if there is none.