Happy Holidays! 
As far as my education regarding Procedural Foliage, and related the UE Grass Tool
A couple of questions:
Regarding the Grass tool, I noticed that in the tutorial example there is a 2 layer landscape blend (Rock, Grass), and each layer uses a single texture instead of 3 texture samples I am used to seeing in a typical landscape material.
So instead of a base color, normal, and say roughness, in this Grass Tool tutorial, a single texture sample (T_GDC_Grass01_D_NoisyAlpha) is used for the grass, and a single texture sample (T_AlpinePatch001_D_alt_R) is used.
Any ideas what the “D alt R” stands for?
For a 2 layer landscape blend in this example, in Landscape paint mode, if I paint rock, rock appears and the procedural grass goes away. If I paint grass, the rock goes away replaced by the grass from the grass tool.
When I look at the texture for the landscape grass “noisy alpha” it give me the impression that it’s the grass that is meant to be seen, not the green of the underlying texture, or why just have a single texture sample as compared to a normal landscape material that utilizes at least 3 texture samples for each texture?
And the rock can be seen, so why is there just a single texture sample used, instead of the 3 texture samples for each kind of surface (rock, dirt, etc)?
Q2:
If I want to place instanced rocks, placed procedurally, would I use the Procedural Foliage Tool or would I use the Grass Tool, both, or neither?
Thanks!