How does Terrain shader works?

Unity Engine
1

I doesn’t mean the basic work (Tex 1-4 in first pass, 5-8 in AddPass, 8-12 in AddPass etc…)
I mean how can I set for example 10 texture in my own script in the shader?
Because I would like to use the Terrain Shader on my Voxel Engine, but I couldn’t find any solution how can I specify the AddPasses and their textures for each.

2

You can download the source of all Unity’s shaders and see for yourself.

3

I’ve downloaded. All it does is inlude the AddPass once. After that I don’t understand
1; How can I set the AddPass’s properties from a script.
2; How can I insert / use the AddPass multiple times, so I would use 10+ textures.
The Terrain script is not Open Surce (as I know)