With the increasing interest to automatically simplify products' 3D models to cope with varying engineering demands, the identi cation of functional designation of components has become an insistent need. In this work, we suggest a method to classify elementary components of a product into a taxonomy of functional designations. This is done based on information present in the product's digital mockup; that is the geometrical properties of di erent solids in the assembly. We argue that relative interactions between adjacent pieces reveal essential information that guides the identi cation of functional properties. We refer to such interaction as conventional interfaces. To allow our reasoning we demonstrate the relationships between geometry and force, and between force and functional properties. These connections establish the link between the mere geometrical representation that we have as input to the desired functional designations of components.