If it starts up it doesn't go down
I reduced the elevation to -1000, doesn't react. is this how it's supposed to behave?
Two screenprints: one with 0 one with -1000
The Simple Shape is just a mask. Here it is masking out a rectangle and preventing the pf node from affecting that part of the terrain. So it's flat.
To use a SS to adjust a displacement, feed it into a surface layer and use that node's displacement options to adjust.
Simple Shape Shader can generate both displacement and color. By default it generates color, which has no "altitude". Technically Displacement doesn't either, it has Amplitude, which is not quite the same thing. It's understandable if that's a bit confusing.
Anyway, here you're using it as a mask, masks are only driven by color, and so it is only the color output of the Simple Shape Shader ("SSS") that has an effect, and all it does as a mask is to confine the displacement *created by the Fractal Terrain* to the area of the SSS. To change the amount of displacement you have to change the values for displacement in the Fractal Terrain node.
If you were outputting displacement from the SSS and *not* using it as a mask (because remember, masks are only driven by color, not displacement), then you still would not control displacement height with the Y value. Instead you would adjust the Displacement Amplitude in the SSS on the Displacement tab.
The existence of the Y input field is kind of a "bug", in a certain sense. The Simple Shape Shader has position, and position is generally represented by 3 coordinates, X, Y, and Z, which is why it's there. But for most situations the Y ("up") coordinate is not really going to have an effect on how the SSS applies to the scene, so arguably Y shouldn't be editable.