My water softener has a setting for KCl (Potassium) / NaCl (Sodium), what does it do exactly?
It controls the weight of the salt that is used.  Potassium has a higher atomic weight than sodium, but they play the same role in the ion exchange reaction in the softener, so you need a greater weight of KCl to achieve the same level of softening.

Potassium chloride has a molecular weight of 74.55, whereas Sodium chloride has one of 58.44, so in theory you should need about 27% more KCl.  In practice, this article cites a study that suggests that less is needed:


  To achieve the exact equivalent capacity of an NaCl softener, the weight of KCl used would need to be increased by 13 percent at the 4 lb./ft.3 dosage level, but by only 2 percent at the 15 lb./ft.3 level.

If your softener is set to use NaCl, but you've added KCl, it'll be less effective than it should be.