In previous versions, there were 3 different approaches to by-products, and recyclable materials were often handled using a cut-off, following the recycled-content approach. That made datasets on recycled materials very simple but also left them missing a key input. For version 3, we introduced a new system model with a new approach to deal with recyclable materials, called Allocation at the point of Substitution (APOS), in which recycling products come with a small fraction of the previous life cycle attached. This can sometimes result in very long, diverse datasets with many, but usually small, inputs, which reflects the diverse supply chains of some waste types.
We have chosen this approach since it provides in our view a more adequate modelling for a background database. Clearly different goal and scope definitions may require other approaches to this issue, so APOS can be combined with other approaches in foreground systems, and we plan to use our new capability for multiple system models to possibly offer alternative modelling approaches here eventually.
More information on the APOS System Model