Great questions:
% solution refers to the percent concentration of carbohydrates, where 10% = 100g carbs per liter of fluid.
Reason I use “% concentration” rather than some measure of energy density: it’s numerically easier and is a perfect correlate of energy density, which tends to be a strong predictor of gut tolerance. Higher energy densities of solution .→ greater chance of GI upset. And since your solution should have >>95% carbs, and no other macronutrients, for optimal absorption the correlation between energy density and % concentration of the solution is a perfect correlation.
Osmolarity = number of molecules in 1 L of solution.
Osmolarity = number of molecules in 1 kg of medium.
I just use osmolarity because it makes more sense in my head.
Osmolarity is harder math for sure, but also matters much less than energy density of solution or % carb concentration in terms of gut tolerance. It’s quite possible to create a low osmolar solution that has very high energy density and end up with gut issues. There are some in my field who believe osmolarity is virtually a non-issue if energy density is managed appropriately. This makes me LOL when I read certain supp companies claims that their gels are iso-osmolar (or some other similar claim implying that their lower-osmolar product doesn’t need water consumed with it).
It’s safe to say that if your % solution is around 10%, the osmolarity will be a minimally important consideration but it never hurts to at least in part, get some of the sodium from sodium citrate, rather than table salt.