The salt mentioned in the Bosch dishwasher manual is used for water softening purposes, even though the dishwasher itself may not have an inbuilt water softener. Water softening is the process of removing minerals, such as calcium and magnesium, from the water supply to prevent the formation of limescale and improve the efficiency of the dishwasher.
Adding salt to the dishwasher helps to regenerate the ion-exchange resin in the water softener. The ion-exchange resin attracts and removes the hardness minerals from the water, allowing the dishwasher detergent to work more effectively and preventing the build-up of limescale on dishes and the dishwasher's internal components.
By adding salt to the dishwasher, you ensure that the ion-exchange resin remains effective in softening the water, which results in better cleaning performance and helps to prolong the lifespan of the dishwasher. The salt used for this purpose is specifically designed for dishwashers and is different from table salt or other types of salt. It is usually coarse-grained and dissolves slowly to provide a steady supply of softened water during the dishwasher's wash cycles.