I just looked at the charts. The nitrite chart says FW & SW so apparently it is good for both. I also just went to their website. From looking at the box photos, the ammoniam, nitrite, nitrate and high pH all say FW or SW on the front while the regular pH says FW. That would make sinse as the pH of salt water is naturally going to be higher and not register on the regular kit that only goes up to something like 7.4.
So I guess the answer is that they can be used for both but the charts may not all be in the FW kit. I only remember seeing one nitrate chart and it says FW on it.