Subsystem engineering is a research field and methodological approach focused on the decomposition of complex systems into distinct, manageable functional units (subsystems) for detailed design, analysis, and verification. It investigates the internal characteristics, performance requirements, and external interfaces of these constituent parts, ensuring their optimal operation and seamless integration within the overall system architecture. Its significance lies in enabling effective complexity management, facilitating parallel development and testing, and enhancing the reliability and robustness of large-scale engineered systems.