These settings are part of func_group (terrain) entity.
If you select a terrain brush and press N to openup Entities window, you can see these values there.
I think that It is from an old method to do texture work on terrain, where you need to blend images between eachother using a .pcx file, and other stuff…
For example, easyGen software can generate terrains like this. The problem is, that you cant really manipulate with terrain and its textures after it is all generated.
I guess you are missing the .pcx file, and maybe some other things that were used for this method (I dont remember it all anymore)…
Maybe you can find it packed in MLB map packs.
You can also delete these entity keys (alphamap, shader, layers, terrain) from func_group, and retexture the terrain by youself in Radiant. I think it should start to compile okey once you delete these keys.
If you go this way, you can also edit whatever part of terrain you want, because it won’t be connected to the .pcx file anymore.
Terrain blending tutorial: