These are buffers which shading system elements, such as the Material nodes and Shading Models, provide.
Most buffers are available here in direct and indirect versions. The "direct" buffers contain data from direct illumination of the surface, meaning the light that came directly from a light source. As opposed to the indirect buffers which contain data from the indirect illumination, that being light that got to the surface via an indirect route such as light that has bounced off another object via radiosity. Some buffers only become available when the materials pertaining to them are used.
Carpaint, Skin and Principled BSDF have several entries in the Extra Buffer list below
Link to Back to Beauty
Show example using linear and srgb for compositing
For most of the extra buffers listed here, there are two additional buffers available, labeled as Rho and Weight, introduced in LightWave 2019. The Rho buffers include the surface's color and any fresnel effects. Weight contains the value of the surface shader setting in this buffer as a percentage of its contribution to the render. A surface that was 100 % dielectric or metallic would have 0 diffuse weight because the diffuse component of the surface makes no contribution to the final render.
The Rho buffer is the BXDF reflection or transmission function that is being evaluated for each microfacet sample. They are intended for heavy compositing work and used to extract, for instance, the color from a specular buffer; by dividing the Rho by the specular direct and indirect to get the raw untinted reflection value.
The Clearcoat_rho buffer provides the amount of Clearcoat occurring based on the way in which Clearcoat changes with glancing angles, fall off, etc.
Clearcoat WeightThis buffer shows the amount that the Clearcoat part of the scene's surfaces contributes to the overall render
The left image shows a final render with a surface HyperVoxel cloud, the second image shows the Legacy Volumetric buffer
For this buffer we changed the dragon to use the Dielectric material
The green glow in this buffer is caused by the luminosity in the edges shown in the final render at the top of the list
Provided by nodes with a Sheen component, is the Sheen contribution from direct illumination
Specular RhoThe specular_rho buffer provides the amount of reflection occurring based on the way in which reflection changes with glancing angles, fall off, and so forth
Provided by the Volumetric system, is the volumetric's contribution from indirect illumination