Non-decorative models, which are often part of a functionally assembly, incorporating so called COTS parts.
Commercial Off-The-Shelf products such as pipes, wires, nuts and bolts made from different materials (steel, PVC, Nylon) help the model gain properties which are not possible with 3D printing. These are additional strength, accuracy, savings in print material and print time, and much more.
Module Source Design Categories should include the COTS materials and categories, at least separate those which use COTS from the rest.
This post should be used to discuss the properties of COTS, and how they might be combined with 3D-printed plastic.