If you imagine a weight-mapped additive layer mapped using a channel, which has 2 bsdfs, one is pure black so it is invisible in this case (it does not add) and one bsdf is lighter. Those bsdf’s are also weight-mapped in other channels, all mapped perpendicularly to each other so that the weightmaps intersect in 3D space.
I will look for a simple example, It has been a while since I have used this.. It was pretty much a workaround for a 3D weightmap in object space. If Bella has a good 3D procedural noise system working in object space, that would be better. I just mentioned this trick as an example of how additive layers can be really useful workaround in a tight spot, and I think subtractive would also allow cunning tricks we haven’t thought of yet.
I often use a multichannel additive approach to add topologically placed dust on dark carpaint, technically this should not be additive, but visually it looks great, and I get control over where the dust goes without needing any UV maps. (this only works well on geometry without overhangs)
When I say subtractive, I am thinking about the behaviour of the multiply layer in Photoshop. I want to take an entire material stack and be able to tint the diffuse output.