Which document can update a system's special instruction according to rules hierarchy?

The correct answer is the system general order because this document is specifically designed to provide overarching guidelines and instructions that can modify or update existing special instructions within a company's operational framework. It serves as a key component of the rules hierarchy, ensuring that any changes or updates are consistent with broader policies and practices across the organization.

System general orders typically address various aspects of operations, specifying rules that need to be adhered to at all levels and can be instrumental in maintaining safety, efficiency, and compliance with regulatory requirements. In contrast, service unit general orders, general notices, and local instructions may pertain to more specific or localized situations and might not carry the same weight or authority in terms of overriding or updating system-wide operational directives. Therefore, when considering which document has the authority to effect change in system special instructions, the system general order is the definitive choice.