This is the amount of gap that there is from the top of the carcass to the edge of the door or drawer face.
On base cabients, top gaps are usually 6mm so there is clearance for the countertop.
On upper and tall cabinets, top gaps are typically 0mm (flush with top), unless you are putting a flush filler piece or panel above the cabinet, in which case you should change it to 3mm.
If you are stacking cabinets, you want to make sure you have a 3mm gap from one cabinet’s doors, to the others. General recommendation is splitting the gap on each cabinet. This would mean a 1.5mm Bottom Reveal on the top cabinet, and a 1.5mm Top Reveal, on the bottom cabinet. This will ensure proper door gaps.