BikeCAD has always provided two dimensions for steerer tube length. One dimension for threadless headsets where the steerer tube extends all the way to the top of the stem and one for threaded headsets where the steerer tube extends to the top of the headset.
In both these cases, this measurement is slightly longer than what you'll need in practice because in the case of a threadless headset, you'll want to cut the steerer tube a little shy of the top of the stem to make room for the top cap. In the case of the threaded headset, the lockring (lock nut) will fasten down on top of the steerer tube, so some extra clearance needs to be added to account for that.
While the extra length of cut needed for a threadless headset is going to be a function of the particular stem used and the shape of its top cap, in the case of a threaded headset it is a function of the headset and specifically the geometry of the lockring (locknut) at the top.
Because this feature request came from builders of Keirin bikes where threadless headsets are still popular, the "Steerer tube extra cut length" dimension has been added to the headset dialog box. I trust this won't be too confusing for everyone else who might associate this dimension more with the stem.