What is a "Synthetic" surface?

[Holland Hacker]

Listening to Hamerle and Steve on ATR talking about Hysterical lady and her lack of ability on Synthetic Surfaces I began to wonder what the definition of a Synthetic surface is.

I know there are several different varities Poly, Cushion, Tapeta and Pro-Ride. Over the last few days it has come out that the new Pro-Ride at SA is 87% dirt and only 13% synthetic. To me this begs the question a what point does track go from being a dirt to a synthetic surface. Obviously it is womerwhere between 0% and 13% synthetic material.

Does anybody know what percent of synthetic material must be present for a track to be considered synthetic?