For certain functions such as the crystals or certain of the voronoi like angles, a negative displacement will sometimes look different than a positive one....sometimes better, sometimes not. It's just a preference. If I have a function that just looks puffy under displacement, a negative turn may look better. I've been using that and negative bump since V4 and 5. Again, just for what looks better.

The foreground terrain mat has an ecosystem on the top portion. The skree material mixed on the rock itself is just the 'pebble noise/sick rock' portion of the terrain mat, mixed according to the functions just like the dark/light color.