If you take a sultana and drop it into a glass of water the sultana will inflate like a balloon. This is due to water moving into the fruit. This is osmosis, the inside of the sultana has a lot of sugar in it and this attracts water into the fruit. The skin of the sultana is a membrane which allows water in, but will not allow sugar out. This creates an osmotic potential and water is sucked into the fruit.
Plant cells are the same, they have a high salt content and a membrane that will not allow salts out (unless the cell chooses to let it out). Water is forced into the cell by the osmotic potential and dissolved ions may be carried along with it, which is what happens during transpiration. Plants may also take up dissolved ions manually, such as potassium, iron. The concentration of the cell cytoplasm is closely regulated for charge and dissolved salts, so if a plant takes up a lot of potassium and it starts to change the salt content of the cytoplasm, the cell may eject another ion to balance the charge, or cytoplasm concentration. So it might eject sodium, for example. This is the counter ion.
Fertilizer solutions high in dissolved salts reduce the osmotic potential and if high enough, prevent water from entering the root cells. This is called salt burn, and why EC is so important. Accumulated salts from failing to flush the potting mix regularly and routinely is another way salt burn occurs. This by the way, has absolutely nothing to do with CEC.
Sodium and chloride are plant micro nutrients, but there is no need for them to be added to fertilizers because sodium and chloride are common everywhere. Even if you use RO water there will be sufficient sodium and chloride in the water for plant health. Many micro nutrients in hydroponic solutions are there because they are selling points, buyers know that Nickel, or chromium, or sodium are considered essential micro nutrients, so they pick products that have them, but most of these exotic micro nutrients are never deficient in plants. In fact it is extremely difficult to induce sodium or nickel deficiency in plants.
Paul