These terms are defined in the OpenType Math Tables as follows :
- StretchStackTopShiftUp Standard shift up applied to the top element of the stretch stack.
- StretchStackBottomShiftDown Standard shift down applied to the bottom element of the stretch stack.
- StretchStackGapAboveMin Minimum gap between the ink of the stretched element, and the (ink) bottom of the element above.
- StretchStackGapBelowMin Minimum gap between the ink of the stretched element, and the (ink) top of the element below.
Please note that this paper does not have a figure for these constants.
I also would like to know what is a "stretch stack".