The X, Y and Z values shown under the 3D preview when you move your mouse over it will generally be indicating the position of the point of terrain under the cursor. When are in camera move mode (holding down Alt and pressing a corresponding mouse key), the numbers will change to "Vx, Vy, Vz", and "Vheight". These are now specific to the camera and indicate the X, Y and Z values of the camera, as well as the camera's height above the terrain.
I'm not entirely sure what you mean by "prescribe a certain height to a terrain when creating a fractal terrain", but if you mean to create features of around a certain height you should adjust Displacement Ampltitude in the Displacement tab. This will set the *maximum* height, so most features will be below that, but used properly it can give you the control you may be looking for. You will have to experiment a bit to see the relationship beween the various scale parameters and the Displacement Amplitude so that you can most effectively specify your height ranges.
- Oshyan
