I believe that the aircraft has to be equipped with a particular piece of kit (the name of which escapes me) for it to show up on a site like this.
Not all aircraft (commercial / military / private) are.
TimC will be along shortly to explain.....
AIS. Except it's not, as AIS is for ships. Something like that anyways. AIS stands for Automatic Identification System so it could theoretically be that.
In a previous existence I got involved in trials of AIS boxes with the PLA. Rock up at the pier, walk on to PLA boat with box of electronic gubbins, spend a few hours going up and down the Thames at Greenwich. Chatting to the developer bloke:
"So have the bugs been ironed out?"
"Mostly. It still has a problem reporting position when it's in the Eastern hemisphere, but that shouldn't trouble us today"
<me shows developer exactly how many times the bendy River Thames crosses the Greenwich meridian>