RO removes most inorganic chemicals (salts, metals, minerals) most microorganisms including cryptosporidium and giardia, and most (but not all) inorganic contaminants.
An RO unit can remove up to 98% of the total dissolved solids (tds) in tap or well water. If your tap water is in the 185 - 215 tds range like mine is the RO filter will get it down to under 10. The DI (De-Ionization) filter removes the rest leaving you with 0 tds. You then use this clean water for tank top off due to evaporation and for your salt mix for a pwc.