2) As some people have correctly said, the salt is used to recharge a built in water-softener (ion exchange resin) in the dishwasher.
3) 'Salt action' tablets do not recharge the water-softener. Instead they contain phosphates. By adding phosphate ions to the water, the calcium and magnesium ions that would normally precipitate out as carbonates (limescale) are kept in solution. Calgon works the same way. The solubility of calcium carbonate in water is a somewhat complicated topic. Seefor an indication.
4) The phosphates, incidentally, have been banned from use in washing powder/detergent as they contribute to eutrophication. Using dishwasher tablets is not 'green'.Regards,
Sid