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On the side of the TIR3 there is a socket that looks like a microphone or earphones socket.

http://www.naturalpoint.com/trackir/02- ... hiteBG.jpg

What can I do with it?

Nothing. It's only for use on the SmartNAV AT and EG models. Since they both use the same shell, the jack is on all models. The TrackIR unit has that port disabled.