Thanks, but Dune is right - i not mean just randomness. I try to explain:
My primary software is Side FX Houdini. I can do anything with geometry or points attributes. For example, i can scale instances by image map, by 4D noise, by sound, by attribute from another geometry, or by anything what you can imagine - is simplest thing that you can do. But why i start to learn terragen3 now ? This is because i'm here: because Terragen is landscape/nature oriented software. It has a lot of features especially for creating rocks, grounds, clouds. And the thing that i can't do in Houdini - realistic lighting, atmosphere, hase, and clouds. Also Terragen has a nice memory management.
But i'm confused sometimes with things like scale by shader. This is so strange

When you can map color to instances, when you can map density, or luminance. But NOT scale!
Look to some my old unfinished "work in progress" Houdini pix. This is a points for trees instances. The bigger trees has a greater distance between points than smaller. This is because red color mapped to points density:

Red points color also linked to scale of trees (image show bound boxes):

This pictures is not for discussion, just example what i mean when say "scale by shader".
Thanks to all.