Hair materials have separate parameters to control specular highlights to better recreate the way light interacts with hair. The highlights on hair materials are not symmetrical and appear to wrap around the surface. The anisotropic (not symmetrical) specular can be used for recreating the look strands of hair or brushed metal without having to model or texture the fine surface detail.
CustomFloat10 controls the amount of anisotropy. This works identically to the anisotropy input of Blender's Principled BSDF. A value of 0.0 produces a round, symmetrical highlight. Higher values produce a more stretched highlight.
Some materials use the PRM alpha value as anisotropic rotation instead of controlling specular. A value of 0.0 is no rotation. A value of 0.5 rotates the highlight by 90 degrees. A value of 1.0 rotates the highlight by 180 degrees. Anisotropic rotation can be used to fix the highlight direction on surfaces with UV layouts that cause the highlight to appear rotated or distorted.
If the following are all true, then the anisotropic rotation map is not required or can be set to 0.0 (no rotation).
If the UV islands for the hair are rotated or CustomFloat10 is > 1.0, the anisototropic rotation map can be used to correct the highlight direct for different parts of the mesh.