A striking feature of complex, open systems is that each comprises a set of highly interdependent sub-systems. For instance, the human body consists of several complex sub-systems (nervous, skeletal, circulatory, digestive, etc) none of which can survive independently of the others. Each of these sub-systems in turn comprises lower-level interacting sub-systems, and so on.
Looking in the other direction, any system can be seen as a sub-system of one or more higher-level systems. For example, the operations, finance and marketing systems are interdependent sub-systems of the firm. The firm, in turn, is a subsystem of other systems: the industry, the economy, the taxation system, etc.