Im not 100% sure about this but i think its the vision blocking hight. Like you cant see over trees for 1 cliff above, but can do it if your 2 cliffs above.
If you set it to 0 you'll be able to see trough the destructable.
The other things, im not sure about, but fat line of sight gives probably a bigger line of sight blocker, since you cant change the sight blocking value.
I got a theory on the "Pathing - Cliff Height", that is when a unit walks over what level he is detected on as, since only bridges got this value and on all of em its set to "2".