Which level in the ISA 62443 Reference Model defines the actual physical processes?

In the ISA 62443 Reference Model, the level that defines the actual physical processes is Level 0. This level is foundational as it encompasses the physical devices and processes that directly interact with the real world. Specifically, it includes sensors, actuators, and the machinery that performs the operational tasks of an industrial environment.

At Level 0, the focus is on the physical layer of the system, including all the tangible elements involved in production and operations. It provides the means for collecting data and performing actions, which are essential for the functioning of higher levels in the model that deal with data processing, management, and security.

In contrast, other levels in the ISA 62443 model, such as Level 1, pertain to the control systems that monitor and control physical processes based on the inputs from Level 0. Levels 3 and 5 cover operational and business planning aspects, including policies, risk management strategies, and system management, which do not define the physical processes directly. Thus, Level 0 is specifically designated for this fundamental aspect of the reference model.