Primitive variables (often referred as "primvar") are data attached to the geometry. This is useful when a shader needs to reference custom data from the geometry such as foam in an ocean shader. There is no need to remap Houdini primvars to RenderMan primvars. All geometry attributes are passed to RenderMan as primvars.
Inside the PxrMaterialBuilder node, we access geometry attributes by creating a PxrPrimvar to read this primvar.
For example, we connect PxrPrimvar's resultF to PxrSurface's Diffuse Gain.