Mari 4.6v4 docs:

Principled BRDF

Control	Type	What it does	Dialog	Notes
	toggle	Toggles the influence of channels connected to the shader without disconnecting them from the shader input.		Displacement is toggled off by default.
Inputs \| Base Color	dropdown	The surface color, usually supplied by texture maps.
Inputs \| Metallic	dropdown	The metallic-ness (0 = dielectric, 1 = metallic). This is a linear blend between two different models. The metallic model has no diffuse component and also has a tinted incident specular, equal to the base color.
Inputs \| Subsurface	dropdown	Controls diffuse shape using a subsurface approximation.
Inputs \| Specular	dropdown	Incident specular amount. This is in lieu of an explicit index-of-refraction.
Inputs \| Roughness	dropdown	Surface roughness, controls both diffuse and specular response.
Inputs \| Specular Tint	dropdown	A concession for artistic control that tints incident specular towards the base color. Grazing specular is still achromatic.
Inputs \| Anisotropic	dropdown	Degree of anisotropy. This controls the aspect ratio of the specular highlight. (0 = isotropic, 1 = maximally anisotropic).
Inputs \| Sheen	dropdown	An additional grazing component, primarily intended for cloth.
Inputs \| Sheen Tint	dropdown	Amount to tint sheen towards base color.
Inputs \| Clearcoat	dropdown	A second, special-purpose specular lobe.		This data is used by the vector inspector.
Inputs \| Clearcoat Gloss	dropdown	Controls clearcoat glossiness (0 = a “satin” appearance, 1 = a “gloss” appearance).		This component is disabled for Ptex channels.
Inputs \| Ambient Occlusion	dropdown	The channel controlling the ambient occlusion effect of the shaded surface. Using this input overrides the ambient occlusion values stored on the object (Objects > Ambient Occlusion).
Inputs \| Emissive Color	dropdown	The channel controlling the color of the emissive (glow) qualities on the shaded surface.
Inputs \| Normal	dropdown	The channel used for input as a representation of the surface normals, added to the shaded surface. You must select either a Bump or Normal shader component in the same shader. If you attempt to use both, Normal overrides the Bump shader component.
Inputs \| Bump	dropdown	The channel controlling the bump map to show as a perturbed lighting.
Inputs \| Vector	dropdown	The channel controlling the vector field and resulting vector data from paint on the shaded surface.
Inputs \| Displacement	dropdown	The channel controlling the displacement map and dynamic tessellation to show a more advanced preview of displacement.
	button	Adds a new channel for the shader component.	Add Channel Dialog
Metallic	text field, slider	The metallic-ness (0 = dielectric, 1 = metallic). This is a linear blend between two different models. The metallic model has no diffuse component and also has a tinted incident specular, equal to the base color.
Subsurface	text field, slider	Controls diffuse shape using a subsurface approximation.		From 0 to 1; defaults to 1.
Specular	text field, slider	Incident specular amount. This is in lieu of an explicit index-of-refraction.		From 0 to 1; defaults to 1.
Roughness	text field, slider	Surface roughness, controls both diffuse and specular response.		From 0 to 1; defaults to .200.
Specular Tint	text field, slider	A concession for artistic control that tints incident specular towards the base color. Grazing specular is still achromatic.		From 0 to 2; defaults to 1.
Anisotropic	text field, slider	Degree of anisotropy. This controls the aspect ratio of the specular highlight. (0 = isotropic, 1 = maximally anisotropic).		From 0 to 1; defaults to .200.
Sheen	text field, slider	An additional grazing component, primarily intended for cloth.		From 0 to 100; defaults to 1.000.
Sheen Tint	text field, slider	Amount to tint sheen towards base color.
Clearcoat	text field, slider	A second, special-purpose specular lobe.
Clearcoat Gloss	text field, slider	Controls clearcoat glossiness (0 = a “satin” appearance, 1 = a “gloss” appearance).
Ambient Occlusion	text field, slider	How much ambient occlusion occurs on the shaded surface.
Emissive	text field, slider	How much glow the emissive channel appears to have.
IBL Quality	dropdown	Performs image-based lighting on the surface of the object. The lower the setting the lower the visible quality but the faster the frame rate. The higher the setting the better the rendered quality, look, and accuracy but the lower the frame rate.
Bump \| Bump Weight	text field, slider	How much weight the bump map has. Lower values are smaller bumps, higher values are larger and more obvious displacements.		From 0 to 10; defaults to 1.000.
Bump \| Bump Mode	dropdown	Displays bump quickly (Fast), accurately (Accurate), or less accurately (Fastest).		While Fastest is very fast, it works better with live procedural layers rather than painted or cached layers.
*Bump* \|Bump Space	dropdown	UV - Normals are calculated relative to the UV coordinates space. The UV Bump Space mode is the more traditional approach but can lead to noticeable seams and color variations. World - Normals are calculated relative to the World coordinates space. Using the World Bump Space mode gives a more seamless result.		By default, Bump Space is set to UV.
Displacement \| Displacement Bias	text field, slider	How much the white or black values are pushed or pulled from the surface.		From 0 to 1; defaults to 0.500.
Displacement \| Displacement Scale	text field, slider	How much displacement is applied to the shaded surface. Lower values equal smaller displacement; higher values equal larger and more obvious displacement.		From 0 to 1; defaults to 0.500.
Displacement \|Displacement Range	text field	What the range of displacement is. This setting is multiplied by the Displacement Scale to give the displacement.
Displacement \| Max Tessellation	text field, slider	How many texels the surface is tessellated to.		From 1 to 64; defaults to 10.
Displacement \|Perturb Normals	dropdown	Selecting yes changes the displacement so that displacement moves the surface but leaves the surface normals as they are.