Shorthand for the levels of the Purdue Model (see “Purdue Model”). In Purdue (which defines a reference architecture for ICS networks), Level 0 is the physical process (sensors/actuators), Level 1 is basic control (the controllers like PLCs), Level 2 is area supervisory control (HMIs, local supervision), Level 3 is site operations (plant SCADA servers, historians, engineering workstations), and Levels 4-5 are IT systems. People often refer to “Level 1 devices” (meaning the controllers) or “Level 3 network” (meaning the control network zone with servers). It’s basically a way to delineate where a device sits in the hierarchy from physical process up to business network.
