Seltzer Versus Soda Water

When I go to the bodega or supermarket to buy some seltzer, which I absolutely love and easily drink a case of each week, I also notice that club soda is sold. What pray tell is the difference between the two?

A) Seltzer, aka soda or sparkling water, is merely plain water into which carbon dioxide gas has been dissolved.

B) Club soda can be identical to plain carbonated water or it may contain a small amount of table salt, sodium citrate, sodium bicarbonate, potassium bicarbonate, potassium sulfate, or disodium phosphate, depending on the bottler. These additives are included to emulate the slightly salty taste of homemade soda water.

So, all club soda is seltzer but not all seltzer is club soda, like how all squares are rectangles but not all rectangles are squares.

Answers courtsey of Wikipedia