The ISA/IEC 62443 cybersecurity standard defines five Security Levels (SLs), numbered from SL 0 to SL 4. 

These levels represent the increasing degree of security protection required for industrial automation and control systems (IACS) against cyber threats:

- SL 0: No specific requirements or protection needed (essentially no security).

- SL 1: Protection against casual or coincidental violation.

- SL 2: Protection against intentional violation using simple means with low resources, generic skills, and low motivation.

- SL 3: Protection against intentional violation using sophisticated means with moderate resources, IACS-specific skills, and moderate motivation.

- SL 4: Protection against intentional violation using sophisticated means with extended resources, IACS-specific skills, and high motivation.

These levels help organizations assess and implement cybersecurity measures based on the potential risk and threat landscape for their systems.