Instead of being applied to the vertex, the renderers of the layers are mapped through 3D space in volumetric; allowing you to control the intensity of each of them thanks 3D position, rotation and scale parameters.
Define a mixture of one / three or six textures corresponding to the X, Y and Z local axis.
as renderer type for 3D Texture, the differents renderer are mapped through 3D Space.
In this case, with 1 uniform with single color for the front, left and top axis:
3D texture act like this:
Thanks this kind of texturing, you are able to control the mapping thanks 3D parameters like this: