The difference between a system and a compound is that while a compound is defined by the sum of its component, a system is defined by the product of the interactions of its components.
This very simple statement has profound consequences, regardless of whether we are talking about chemical, physical, social or entire economic systems.
Decomposition and Reassembly
Classic science has it that if you de-compose a complex problem into smaller units, the complexity can be handled in individual bites. While this works great when interactions are not as prevalent, it entirely fails when the behaviour of a system is predominantly defined by component interactions.
A de-composed system missing even one of its interactions will not display the same properties as the complete system.
Modifying a de-composed system may create an entirely different system when re-assembled.
Interaction generates friction. The mechanism of minimizing friction is synchronization.
As friction reduces the motion energy of the affected components, the amount of friction gradually reduces until the interacting components will have minimal friction. As such, every interacting component of a system will enter into a synchronized state over time.
The momentum of a system in a synchronized state will be the cumulative momentum of all components. The same holds true for inertia.
Synchronization does not equate stability. Indeed, the process of synchronization could destabilize, and potentially destroy, the entire system.
On a higher level of abstraction, a subsystem behaves like a component, assuming its internal and external interactions are separate and distinct.
Interacting subsystems will generate friction until they are synchronized.
Subsystem synchronization could oscillate between different states and have different driving forces until an equilibrium is achieved.
Independent subsystems behave like components: they may be in sync within themselves, yet out of sync with each other.
Why is any of that important?
Think about what the above means for piloting changes in parts of your system.
You may not achieve what you intend.