When you make some land form or adjustment with Wilbur you often have an option about how to apply effects: Replace, Add, Subtract, Multiply, Divide, Min, and Max. When I try things out, changing these options, I don't usually get what I expected. Like when adding fractal noise I expect the new noise to be applied on top of the existing land, so the heights from the new noise rise up from the pre-existing landscape. But it doesn't seem to work that way.

What do these options really mean and how do they apply effects?