OT systems occupy the lower levels of the model while IT systems occupy the upper levels, with a “demilitarized zone” of convergence between them. Let’s take a look at each of the zones in the Purdue reference model, top to bottom: Level 4/5: Enterprise Zone Level 3.5: Demilitarized Zone (DMZ) Level 3: Manufacturing Operations Systems Zone Level 2: Control Systems Zone Level 1: Intelligent Devices Zone Level 0: Physical Process Zone Read more.