A system of systems is a collection of many different systems, each with its own characteristics. 
System theory assumes that certain underlying concepts and principles are applicable to different areas, although each field has developed for itself. The main assumption of system theory is that complex systems consist of several smaller systems, and that it is the interaction of these small systems that produces the complex system as we know it. A whole that is more than the sum of its parts is simply a system with constitutive features that can be explained by its internal structure and its interaction with other systems. 
 D. Hitchins, “SYSTEM OF SYSTEMS – THE ULTIMATE TAUTOLOGY? | Systems World | Derek Hitchins,” Feb. 20, 2009. (accessed Jan. 27, 2021).
 “What is Systems Theory? – Social Work Theories & Approaches,” Jul. 2020. (accessed Jan. 27, 2021).