Security Levels (SLs) are a way of expressing the security performance of an industrial automation and control system (IACS) or its components. SLs are broken down into three types: target, capability, and achieved1.
* Target SL is the level of security performance that is required for a system or component to protect against a specific threat scenario. The target SL is determined by conducting a risk assessment that considers the likelihood and impact of potential security incidents1.
* Capability SL is the level of security performance that a system or component can provide based on its design and implementation. The capability SL is determined by evaluating the security functions and features of the system or component against a set of security requirements1.
* Achieved SL is the level of security performance that a system or component actually provides in its operational environment. The achieved SL is determined by verifying that the system or component is properly installed, configured, maintained, and monitored1.
References: ISA/IEC 62443 Standards to Secure Your Industrial Control System, page 3-4.