RO should remove 98-99%+ of TDS from tap water, but will still contain residual TDS.
The DI stage will remove the remaining TDS (basically removing what the membrane could not), producing 0ppm RO/DI water.
As an example with a 98% rejection rate, the RO membrane should reduce 350ppm-TDS tap water to 7ppm RO water. DI resin will then reduce that 7ppm RO water to 0ppm RO/DI water.
One thing to consider, if your tap water is treated with chloramine (a chlorine+ammonia disinfecting compound): the carbon block or catalytic carbon block stage will unbond the ammonia from the chlorine and adsorb the chlorine. The carbon block and RO membrane cannot filter out the remaining ammonia, however the DI stage can.