Understanding that diffuse lighting is just small scale sub surface atom and light intermingling was a revelation to me XD



Documentation→Reference→Ingredients→Surface/Diffuse
The diffuse ingredient scatters light, and colors it by the surface albedo.

Its type can be changed between The Greatest Sandwich, Unity Standard, Standard, Microfaceted, Translucent and Light Wrap by using the side arrows next to the preview image.

The diffuse ingredient can have custom lighting enabled by toggling the two buttons on the top right - the light bulb enables custom direct light, and the indirect lighting box enables custom indirect light. Custom lighting can either be completely overridden, or altered using layers with the type Adjustment.
Global options:

Light Radius - Sets the light radius...

Use Ambient - When off ambient and indirect lighting is disabled.

Use Tangents - Instead of using the normal vector (that points directly out of the surface) for lighting calculations, use the tangent vector (that points along the x-axis of the texture) instead - can be useful for hair and grass if used right.

The Greatest Sandwich ...shader model
Creates very convincing rough surfaces - even roughening indirect diffuse; smoother ones look very similar to Unity Standard.
Smoothness/Roughness - How smooth/rough the surface is; a rougher surface has more microscopic groves and dents.		 Unity Standard
Unity's standard brdf.
Smoothness/Roughness - How smooth/rough the surface is; a rougher surface has more microscopic groves and dents.
Standard
A very simple lighting model with no roughness options.
Disable Normals - Lowers the effect that the angle between the surface and the light has - at 1 it is purely attenuation based.		 Microfaceted
Once again, is much better at rougher surfaces than Standard - however this one is deprecated in favour of The Greatest Sandwich type, since it overly darkens and is less optimized.
Roughness - How rough the surface is; a rougher surface has more microscopic groves and dents
Translucent
A very cheap yet convincing sub surface scattering approximation.
Light Wrap - How far the SSS wraps around the surface.
Color - The color underneath the surface.		 Light Wrap
Yet another translucency approximation, this one even cheaper with still rather nice results.
Light Wrap - How far light wraps around the surface.