As we talked with school teachers about the use of physical computing systems in class, one major drawback of these systems became obvious: almost nobody--next to enthusiastic autodidacts--has been able to tell us on an abstract basis for which educational purposes a certain physical computing system can be used, i.e. in a goal-oriented way. This insight lead to the approach to structure the field of physical computing systems with the overall goal to make it easier for teachers to improve their lectures due integrating physical computing systems into computer science education. The aim of this paper is to structure the field, to find categories and to show how existing devices can be integrated into the founded structure.