A by-product is a Product output from an Activity that is not a Determining product. This means that changes in demand for the by-product do not directly influence the production volume of the activity from which it originates. Instead, the production volume is determined by demand for other product outputs from the same activity.
According to ISO 14040, co-products are defined as "any of two or more products coming from the same unit process or product system". Within this broader category, by-products represent the dependent products whose production levels are determined by the activity's determining products rather than by direct market demand for the by-products themselves.
A key characteristic that distinguishes by-products is their displacement potential. Product outputs that can displace Determining products from other activities are always classified as by-products. This displacement capability makes them valuable in Life Cycle Assessment modelling, as they can substitute for products that would otherwise need to be produced through separate activities, thereby avoiding the environmental burdens associated with that separate production.
Materials for treatment present a special case in classification. Whilst these materials can technically be classified as either by-products or Wastes, this distinction has no practical implications for LCA modelling. What matters most is understanding the material's role in the system and whether it can provide value through displacement of other products or requires treatment services.
The term "dependent Co-product" is used synonymously with by-product, emphasising that these products are dependent on the production decisions driven by other, determining products from the same activity.
