RE: Co Product Vs By Product
Co-products are typically made at the same time and are planned together thus a demand for either triggers a supply order for both.
A by-product is typically you don't wan to plan to make but can't; avoid e.g scrap or offcut or second grade material.
As example you might injection mould parts, and a given mould might product left hand and right hand shapes at the same time. You may get demand for both in equal quantities, or each may be ordered individually e.g as spare parts. So when you make A you het B and when you make B you get A so when you get several demands for each the system has to iterate to decide the optimum order size that will produce both together for the minimum quantity that will satisfy both demands.
However there will also be an unwanted sprue which can be reground and recycled and thus have value. That by-product will not be planned on its own, but material for its manufacture needs to be provided and the cost of recovery material offset.
In such cases the amount of by-product expected be generated can be estimated. However you may also get damaged parts of A and B that can also be received as by product.