The majority of resources, like time and costs, consumed in industrial product development can be related to detailed design, i.e. the materialisation of machine parts (German Maschinenteile). Existing design theories based on a systems approach, e.g. Haberfellner  all have function, i.e. transformation from input to output or ability to deliver purposeful effects as the core concept. The units in a product which posses functions are the organs (German: Funktionsträgern). Because individual parts do not posses functions, one could argue that the design theories based on a systems approach are not valid for detailed design. As a consequence of missing theory for detailed design, there exists only sparse methods in design methodology supporting detailed design. The purpose of this paper is therefor to contribute to a design theory valid for detailed design. A first step towards such a theory seems to be identification of a purposeful behaviour concept, i.e. function for a machine part. The contribution is based on the theory of technical systems, Hubka and the domain theory, Andreasen.