It's the bracket looking thing on the side of the Bryce terrain canvas. In Bryce 5 it is hidden so you have to click on the icon on the top of the right corner of the canvas window to get it to show. You just click and drag the top or the bottom of the clipping tool up or down, depending on if you want the top or bottom removed. This is also good to remove the top or bottom from any other tweaking, like adding noise, then unclip to see it again.