Closed systems receive no inputs. They are either dead or a sort of perpetual motion machine. Boundaries separate the system and the environment. The issue of boundaries addresses the question of what is the system and what is the environment. Where does one end and the other begin.
Boundary specification is important to considerations of control.
If something is within the boundaries, it is under control. Outside is not. The system must design strategies to address opportunities and threats in the environment. Feedback is a source of inputs. This suggests adjustment and self-regulation.