This node vertically stacks 2 materials, with the base being at the bottom of the stack, with the Top layer above it.

Material Top

Material to be used as the top layer.  If no top layer is set, then the base material will be used by itself.

Material Base

Material to be used as the base layer 

Top Mix

Defines the mix between the base and top layers.  This value can either be set via the slider or B/W map, PxrPattern, or float value.

Top Thickness

This value defines the thickness of the top layer. This value can either be set via the slider or B/W map, PxrPattern, or float value.

It is only relevant for interior effects associated with the top material, such as absorption. If the top material is itself a layer node, this value is passed on to its base component. And if the top material is a mix or add, this value is passed on to both child materials.