The stack height is the vertical space taken up by the headset. With a conventional threaded fork, the steerer tube is longer than the head tube by some amount. If a headset is chosen with a stack height greater than this difference, there may not be enough of the steerer free above the upper bearing race for the keyed washer and lock nut to be installed. In the case of threadless headsets, the stack height also includes the height of the part of the handlebar stem that clamps to the steerer. ---Taken from SheldonBrown.com

One last question,I just want to verify.To get the stack height, I measure from the bottom of my headset,to the top.. right? Or just the head tube hieght? I think my first assumption is right. And looking at the article closer,it seems like I need to measure the stack height, and the steer tube,and then subtract the measurments, and my result is the stack height.