IB: SSF settlement?

Are SSF's settled in the underlying stock or cash on expiry? Do I have a choice? Couldn't find the answer on IB web site.

If there are any gotchas in SSF's please tell before I make my first trade ... or should I just stay with plain stocks.

 

um..no volume

 

http://www.onechicago.com/tutorial/index.html

 

Thank you. I read through some OneChicago materials but managed to miss that.