When polyester is labeled 600D, it is a reference to the denier or fineness of the material's fiber. Denier is used to evaluate nylon, silk, polyester and some other artificial fibers. The denier number refers to the weight in grams of 9,000 meters of the fiber in question.
Fibers with lower denier numbers are more fine than those with high numbers, and fibers with higher numbers are stronger. However, it is not possible to measure denier across fabrics. In other words, while a 600D polyester fabric is stronger than a 450D polyester fabric, it is not as strong as a 450D nylon fabric, because nylon is inherently a stronger fiber than polyester.