I used to know what the difference was, but knowledge of that was lost when I started going to college. Go fig.
Anyhoo, it's the coolant temperature sensor that screws into the steel heater lines. Why they couldn't have just gone with stainless lines is beyond me (oh wait, it's not, they wanted to make it cheaply!).
If I were to stab in the dark at the difference, I'd venture to say it's for a secondary temperature sensor. Mebbe they figured it'd come in handy in the future? Who knows. Kind of the same reason there's fog light wiring present on most fox-body Mustangs, even though there are no fog lights. It's cheaper to make everything the same on every car, which is why you can hardly get ANY options worth while on cars anymore. Anyone remember back when bumpers were optional?
