The Clear Coat channel simulates the effect of a thin reflective layer on top of a surface. A common use case is the transparent coating on top of car paint. It can also be used for things like varnish on wooden furniture, wet surfaces, or anything that benefits from having multiple layers.
You can control the coat's Intensity (grayscale texture or single value), Thickness, Reflectivity, and Tint (solid color). You can also add surface Roughness (grayscale texture or single value) and a Normal Map.