Cation exchange resin is also known as cation deionization resin or water softener resin depending on how it is chemically charged.
Cation (Cat-Ion resin in the sodium (Na- form is used to soften water by trading calcium (ca and magnesium (mg for sodium (na . Cation resin in the hydrogen (H form the resin is used to trade all positively charged ions in water for hydrogen. (when used with Anion Resin (OH it makes pure water. H_OH = Pure H20.