gradient of 0.25 is ground which is nearly flat
gradient of 5 is ground which is very steep (steep side of a mountain sorta thing) it means the terrain is going up/down by 5 blocks for every one block you move across
depth is how thick the ground beneath the instance needs to be to consider spawning it
clearance is how big the gap to terrain above the instance needs to be to consider spawning it.
all these paramteres are based on the 'position' of the instance, so they will start to fall down for instances that are wider than a few blocks