Passive Anti Theft System. I did a quick internet search and found this article from what looks like a programming company. Check it out.

http://www.bsecorp.com/files/techtips/FORD%20PATS.pdf

What I have learned from reading is the system stops the start signal somewhere between the key signal to the PCM. If PATS is not satisfied with the security the system shows open on a relay and does not allow the start signal to complete the cycle. When the security system is satisfied the system shows the relay closed and the signal completes the path and starts the vehicle. Shortened version I guess you could say.

SS