What is the Contribution % in the Nutrition section of a recipe?

At the bottom of the nutrition section for every recipe is the Component Nutritional Values table, which shows the breakdown of the calculated values across all recipe components.

In that table is the Contribution % field, which allows you to specify how much of each component’s nutritional value contributes to the finished recipe. This lets you account for the processing loss of an ingredient or sub-recipe. For example, a marinade might only contribute 10% of its nutritional value to the final recipe, despite the calculated nutrition value being much higher. 

To edit this value, go into the "Nutrition" tab of a recipe. Then, scroll down to the "Component Nutritional Values" table and edit the "Contribution %" column.

Adjusting the contribution % of a recipe's component will decrease the contribution of that component's nutritional values uniformly. Additionally, it will inform the ingredient list sort order on nutrition labels along with the ingredient weight.