What term describes any system that is part of the target system?

The term that describes any system that is part of the target system is "subsystem." A subsystem is essentially a smaller, self-contained system within a larger system. It can operate independently but also interacts with other subsystems to contribute to the overall functionality of the main system. This highlights the hierarchical organization of systems, where the main system is made up of various subsystems, each playing a specific role in achieving the overall objectives.

By contrast, while "component" and "module" may also refer to parts of a system, they are less specific in indicating that they are smaller systems operating within a larger context. A "unit" could imply a fundamental part, but it does not convey the same structured relationship as a subsystem. Therefore, choosing "subsystem" accurately represents the relationship and function of the term in the context of systems theory and design.