Security Levels, or SLs, are an IEC 62443 way to express how much protection an industrial system, zone, or component should have. The scale runs from SL 0 to SL 4, with higher levels requiring stronger resistance to more capable attackers and more demanding security controls. An SL is not a product label; it is a target derived from risk, threat assumptions, and the role of the asset in the OT environment.
Security Levels matter because they turn broad security goals into engineering requirements. In practice, teams use them when designing zones and conduits, selecting controllers or remote-access tools, and checking whether a supplier’s controls meet the expected protection target. They also help auditors and integrators compare what is needed, what is implemented, and where gaps remain. In real defenses, an SL target can drive segmentation, authentication, secure maintenance, logging, and patch handling; in attacks, weakly protected assets often fail to meet even a basic target because they lack segmentation or hardened access paths.