Ok, not complicate this, but:
The vast majority of what is lost is just water. Water evaporates, salt is left behind. If a tank is left long enough, and see a good percentage evaporate, the salinity increases relatively proportionally.
However,
Some salt is lost, as well. This is the salt creep that we all have to pick off the sides of the tank, cords, and anything hanging off the tank.
So while the majority of what is lost is water, there is some salt lost, but its inconsequential, and replacement should be done with pure RO.