Thank you Ami that was a nice and useful explanation. I will be the first to get that book :)

I was actually trying to get features in a terrain not textured but actually "carved" in the terrain, but I have encountered problems. Imagine a dune (procedural of course), I want to create cracks in it. In the Altitude production function, I have a simple fractal generating the dune, and a noise node set on Cellular->Drought.

Both feed a Blender combiner node that goes into the Altitude.

This way I have nice cracks all over the dune, but I want the cracks to only appear at the bottom of the dune and not at the top.

I thought the best way to do that was to extract the blender ratio parameter and connect it to an Altitude input, following the examples on pages 340 and 404/405 of the manual. Unfortunately it doesn't work and the cracks keep appearing all over the dune. Adding a Brigthness-contrast filter helps a bit but I still get cracks at the top. If anyone has any idea that would be welcome.