In the interdisciplinary work of computer scientists and mechanical engineers on integrating IT components into physical products and creating prod-ucts-services-systems, we observed that design and development processes cross-cutting the traditional boundaries of disciplines reveal unexpected differences in seemingly easy-to-define and understood terminology. More concretely, at first glance, the terms module and interface have the same meaning in both disci-plines. Still, substantial differences became apparent when implementing design and development processes for products that extend a traditional physical product by IT-controlled functionality and customer services. Motivated by an example of quantified product design, this paper analyses commonalities and differences between the meaning of module and interface in the involved disciplines. We propose an integrative definition with the term "interface" in its focus.