The purpose of the XL Option is simply to give you the choice of saving some money on the two refill items you will likely need first. Generally, this option is intended for owners who are having problems with their water clarity, especially due to algae, and who will need to do quite a bit of FAS/DPD testing. If your water is clear and you don't test often, you don't really need this. It is also ideal for owners performing a Baquacil conversion because of the high chlorine levels you will need to test. This set includes 10g of R-0870 DPD Powder and 2 ounces of R-0871 FAS/DPD Reagent.