Tortoise is Yikun’s overmould plastic, used on the brand’s two-piece discs: a firmer rim and a different flight-plate material in one disc. Two moulds are listed in it.
Overmoulds are made for grip and stability at once, and Disc Golf Dojo doesn’t rate the class on a single card because there’s no single material to rate.
It’s a specialist plastic. The rest of the Yikun range is in Dragon, Phoenix, or Tiger.