An RO membrane only removes a percentage of the contaminants in water, this is what they are talking about when they refer to a rejection rate. Most membranes have a rejection rate in the mid 90s, so lets say you have 200 ppm of phosphate and have a RO unit with a 95% rejection rate, you will still have about 10 ppm of phosphate. This is where the DI cartridge comes in, it removes more than an RO membrane can but would become exhausted very quickly if you tried to use it to remove everything so by having it filter water that has been filtered through an RO filter it has much less to remove. By combining the RO and DI together you get water that should have a TDS of near 0 which is what you want.