Yes, it is the child seat tether, but it has another use?
Officially it is there to hold the child seat tether on it's way down to the tether clip point.
But, I use mine as a place to hang my drycleaning?
Slide the seat forward and open the roof (cabrio) and your cleaning will hang behind the seat. Then slide the seat back and you don't have to worry about the plastic bag inflating while you're driving with your top down?