They look like composites, but these boards are actually 99 percent virgin PVC plastic, very different stuff from the polyethylene-wood mix found in most composites.
PVC is harder than polyethylene, so it doesn't scratch as easily. And because it doesn't contain wood fiber, it won't absorb water or food stains. (Suntan lotion and insect repellent will stain it, however.)
But all-plastic decking has a very limited color palette—three or four colors at most—and it's pricey, about the same as a top-of-the line composite. Plus, PVC is not particularly friendly to the environment: There's no recycled content, and when it burns it releases toxic hydrogen chloride gas.