The 3Delight Principled material is our interpretation of Disney's Principled BRDF (see References). Quoting from the main reference, a principled material adhere to these principles:

1. Intuitive rather than physical parameters should be used.
2. There should be as few parameters as possible.
3. Parameters should be zero to one over their plausible range.
4. Parameters should be allowed to be pushed beyond their plausible range where it makes sense.
5. All combinations of parameters should be as robust and plausible as possible. We thoroughly debated the addition of each parameter.

3Delight Principled shader has more than the 11 parameters listed in Disney's reference as it offers more control on the coat and also has subsurface parameters.

Coating is an important part of a car paint as it affects the overall look in three main aspects:

1. It provides a richer specular response to light as it adds a specular highlight on top of the base surface's highlight.
2. It can colour light as it passes through the coating.
3. It contains metallic particles (a.k.a flakes) suspended inside the coat. 

 
→ Thickness

The thickness of the coating layer. The thicker the layer the more it absorbs light and the less of the base layer is visible. A thickness of 0 disables the coating 

→ Specular Level

How "shiny" is the coating. A value of 1 means a very shiny coating.

→ Color

The colouring producing by this coating layer. Colouring will be more prominent in thicker layers.

→ Roughness

Roughness of the coating layer.

→ Anisotropy

→ Subsurface

→ Color

→ Scale

→ IOR

→ Color

Colour of the emitted light.

→ Intensity

Intensity of emitted light. Final colour is Colour * Intensity.

* References

• Physically-Based Shading at Disney by Brent Burley, Walt Disney Animation Studios.