The salt is used in the Water Softener ("Ion Exchanger") which is built-in to the machine. I have a Water Softener for my entire house so I actually have the dishwasher's built-in softener switched OFF and therefore never have to fill it with salt.
Obviously, that just means I have to fill the house Water Softener with salt instead.
A few years ago I realised that here in the UK, we now pay 20% VAT (tax) on Water Softener salt but not on table salt because it is deemed a food product and not liable for the "luxury" tax. I decided to try using a bulk bag of catering salt in the softener.
I first asked the manufacturer of the softener if it was OK to do it. They recommended NOT to, but could not really explain why. I tried adding about 10% of the granular salt to each refill with the large tablet salt I normally use.
I have no idea why, but the machine started to behave strangely after a few fills with the table salt even though it was quite a small proportion each time. The softening action wasn't working and a sort of brown foam started appearing inside the softener. Also, the softened water started to taste of salt.
Hence, I stopped using the culinary salt in my softener once that bag was gone and I suggest it's not a good idea to try it in a valuable dishwasher.
The one tip I can recommend though is that you actually buy Water Softener salt for the dishwasher. It is exactly the same thing as you get in small bags from the supermarket - but you buy it in 25Kg bags for around £7 - probably around a fifth of the price you're paying at the moment. You just need to be careful because softener salt come in a variety of forms: granular, large granules (about 5mm), tablets (about 20mm-40mm) and blocks. I would have thought that either of the granular forms would be OK in a dishwasher.