It means if you are using rainwater or reverse osmosis water (RO water) your water will not have any minerals. He is asking what Calcium and Magnesium people add to their water supply - and at what rates.
Since I live in Southern California, our water has so much calcium in it you can almost walk on it
However I do supplement my trees with magnesium in the form of magnesium sulfate, particularly in the summer when salt levels in our irrigated water are high. I would have commented, but since I don't actually add supplements directly to my water, I'm not sure it would be the information the OP was looking for.
[EDIT] Crust and I posted at the same time. [/EDIT]