Separation of duties is a security principle that aims to prevent fraud, errors, conflicts of interest, or misuse of resources by dividing critical tasks or functions among different people or teams. It is one of the foundational requirements (FRs) of the ISA/IEC 62443 standards for securing industrial automation and control systems (IACSs). According to the ISA/IEC 62443-2-1 standard, separation of duties includes the following system requirements (SRs):

SR 2.1: Security management policy

SR 2.2: Personnel security

SR 2.3: System development and maintenance

SR 2.4: Incident response and recovery

SR 2.5: Compliance and review

Among these SRs, the one that is most related to the example of system development and maintenance is SR 2.3. SR 2.3 requires that the IACS shall provide the capability to ensure that the development and maintenance of the system and its components are performed in a secure manner. This means that the IACS should have a mechanism to control the access and authorization of developers, testers, integrators, and maintainers who work on the system and its components. It also means that the IACS should have a mechanism to verify and validate the quality and security of the system and its components before, during, and after the development and maintenance processes.

Therefore, an example of separation of duties as a part of system development and maintenance is that changes are approved by one party and implemented by another. This ensures that the changes are authorized, documented, and reviewed by someone who is not involved in the implementation. This reduces the risk of introducing errors, vulnerabilities, or malicious code into the system and its components.