What "makes" a 66 Barracuda a Formula S ??

and possibly a deep pocket if it's all original.
Are they really worth alot of money?
I passed on a running 68 formula S 340 auto, the widow ended up giving it away.
It was a pretty clean car, but I just did not need it.
The guy that ended up with it, flipped it, the car was last seen at Carlisle with a 10K price tag on the windshield.