water is further purified FW
(if you will). You are trying to relate to terms in a way that is incompatible with their respective definitions.
Think of it this way.
Freshwater : Water without salt added to make salt water (be that tap water, well water, RO
Saltwater : Freshwater with salt added.
Water : Freshwater that has been further filtered (not to be confused with filtered water) to remove all solubles making it pure H2O and nothing more. No addtiives (iron, chlorine, nitrates, phosphates , etc).
You use Freshwater to make both and RO
is preferred for saltwater tanks.,