The specification of interfaces is critical in modularization and product architecture development. Literature defines product architecture as (1) the arrangement of functional elements, (2) the mapping from functional elements to physical components (3) the specification of the interfaces between interacting physical components. However, other scholars state that interfaces should include more than physical components, such as spatial, material, energy, and information exchange. This view has been extended to include attachment, transfer, control and communication, power, spatial, field, and environmental interfaces. However, to use interfaces through the product lifecycle and reuse them between product architectures and generations, there must be an approach to handle applicable interfaces in a company. This research contributes by presenting a way to operationalize (investigate an abstract concept, it's essential to make it measurable and tangible) interfaces by introducing interface requirements that are definable, measurable, definable, and testable properties as a part of the interface development process and interface description. The method is illustrated by applying it in an industrial case study.