Reading TraceRes.SurfaceProps

I need to read the surface properties of whatever a trace hits. I found this in the notes for update 153:

Sure enough, the field is there, and it’s usually a double-digit number like 28. I don’t know how to read the number or what it means. I’m assuming it’s a composite of binary flags but I can’t find any info about it.

Any knowledge about this? Garry says it was requested, so somebody must have implemented it somewhere.