Different type of Radars

HI everybody,

setting up radar filters I've noticed that Radars in the list (for making the radar selection) are of three types:

F-xxxx
N-xxxx
T-xxxx

I suppose that F-xxxx identify the locations hosting FR24 receivers (I mean devices provided directly by FR24 network, used around the world but of Fr24 property); but what about the other two?

Any idea what are N-xxxx and T-xxxx radars?

Thanks for your attention.

Ramon Llull