By default, (for example), in the height section. There's a scroller for the height you set the terrain to. It only goes from -3000 to 3000. The true limits are -32768 to 32767. when you set a value higher than 3000, the scrolling "white square" ventures outside of the scrolling "bar".
When placing units in the objects section, there's a scroller that has a limit from -3000 to 3000. I feel that the limit should be increased, especially since if there's a limit, it's a lot higher or lower than -3000 or 3000.