An inter-personal relationship forms a system. There are certain principles and mechanics at work that are different from the workings of an individual. If one doesn't notice that, that domain might frequently end up in a crisis. There are certain principles and issues that might apply to all inter-personal relationships. There will also be different conditions and different agreements from one relation to the next.
Any group forms a system in itself.
Its principles and cycles often operate beyond the activities of individuals and inter-personal relations, and might well continue even with totally different people as participants. There are models and techniques that facilitate working with groups in general, and there are also unique cultures in different groups that must be accommodated. Societies are systems. Planets are whole systems, even though they contain many sub-systems. There will be certain principles that are generally useful for managing planets, but each planet will also be different.