A control system usually consists of some control software as well as sensors and actuators to monitor and control certain quantities in the environment. The context of the control software thus consists of the sensors and actuators it uses and the environment. When starting development of the control software, its context is often not predefined or given. There are contextual decisions the developers can make (e.g. which sensors/actuators/other systems to use). By means of these decisions, the context is defined step by step. Existing approaches (like the Four-Variable Model) call for documenting the environmental quantities (monitored, controlled, input, and output variables) that are relevant after making these contextual decisions. The environmental quantities that have originally been relevant (i.e. before deciding which sensors/actuators/other systems to use) are not documented. This results in problems when the software shall later on be reused in another, slightly different setting (e.g. with additional sensors). Then, it is hard for developers to decide which environmental quantities are still relevant for the software. In this paper, we suggest an extended version of the Four-Variable Model, the Six-Variable Model, and, based on that, a context modelling method, that combines existing approaches. The benefit of our method is that the environmental quantities that are relevant before and after decision making are documented as well as the contextual decisions themselves and the options that were selectable. In this way, later reuse of the software is facilitated.
