This is down to what is passed from your web server to PHP. If there's nothing there, then either it didn't pass anything down (or at least pass it down in the expected format).
This section is really more about debugging than anything else, often to make it easier for us to get particular details when trying to provide support. You don't really need to pay much attention to it for the most part unless you have problems. (And even then, it would generally involve you being asked for particular info.)