The next step in product description models was to introduce a hierarchical structure to the parts list to keep control over the evolving number of parts. But during the 70’s and 80’s products in some industries started having so many variants that it became too tedious to update each variant of each model as a separate hierarchical product structure. Companies started using labels to describe the usage of sub-assemblies that were alternatively used in different variants of the product.
In modern product description the whole structure is parametric hence configurable. The elements are abstract representations of design solutions and will only represent physical parts or assemblies when they are configured through assignment of values to the necessary parameters. The driver for the change from a parts hierarchy with variants to configurable structures is usually attributed to mass customization.