Originally Posted by Greenfieldja

Absolutely correct, even Campagnolo referred to the hidenset headset design with the longer conical spacer as TTC or tall top cap, so your confusion is totally understandable. As others have stated, what is commonly referred to as a top cap for a threadless stem system is the piece that is placed on top of the stem used to adjust the bearing preload of the headset. The top cap pulls the fork up and pushes the stem and spacers down to set the preload and the the bolts on the side of the stem lock the adjustment in place. There needs to be a slight gap (+/- 5mm is recommended) between the top of the steerer tube and the top of the stem to allow the top cap to do its job...if it contacts the top of the steerer tube it may not allow enough compression to properly adjust the headset.

-j