how to distinguish the desired output(s)
In case of Porridge preparation. The 3 servings of porridge I would consider desired output, the one that motivates executing the process. Peals and gray water come also as output but they didn't motivate that process.
IMO we need a way to distinguish those outputs. @bhaugen mention during recent chat 'recipe for product' where in this case we produce '3 servings of porridge', we don't usually make recipes for 'gray water' or 'fruit & vegetable peals'.