For example, you may use a brine to make corned beef, and throw away what's not used of the brine. So the brine is a cost. But only so much is absorbed in the meat (i.e., the nutritional amount). reciProfity will now calculate the amount of brine absorbed based on the prep % so that the nutritional label does not display too much sodium (salt) on the label.
In the recipe, in the Prep column, indicate how much of this ingredient is used in the nutritional.
For example, n%45 in the prep column means use 45% of this ingredient in the recipe as a nutritional amount.
IMPORTANT: This special nutritional prep does not stick to the ingredient. If you want it to stick to the ingredient then add it to the prep column in the Conversions section of the ingredient.
Then when you use it in a recipe you can choose the ingredient with the correct Prep. Below is an example of how to choose an ingredient with a nutritional prep that displays the correct %: