Are there some forms of sodium that should be avoided and some that should be consumed? When will sodium hydrate you and when will it dehydrate you?
prefer sodium chloride (aka salt), avoid the others
sodium alone is not responsible for hydrating/dehydrating, it’s more complex than that. suffice to say that industrialized edible substances are to be avoided if you are concerned with water retention. in their absence, thirst is a remarkably effective regulator of hydration
If there is water available in the digestive tract but the blood salinity is too low then it cannot be absorbed. At that point consuming a small/moderate amount of sodium is useful. The body has an active transport system that can bring sodium into the bloodstream. Once there, water can be absorbed from the digestive tract via osmosis. If the blood sugar level is too low this won’t work, however, so including some glucose can be useful to power the machinery but it is better if the glucose is already there. I’ve run into this on very long, hot, workouts. I let my sodium level drop too low while also depleting glucose. Stored fat can power muscles but not sodium transport.
Too much sodium can exceed the capacity of the active transport. Then it just raises the salinity in the digestive tract. If it gets high enough, osmosis draws water out of your body into the bowels instead of the other way around.
As others have mentioned, this is most commonly seen in athletes. However, low blood salinity can also show up when diarrhea-like conditions are present. This can be from a transitory illness or a chronic condition. IBS-D fits into the chronic category.