I’m sure most people are familiar with aquariums, salt or fresh water containers filled with flora and fauna plus various components to keep the aquarium alive: heaters, pumps for aeration and filtration, light, pH controllers and more. An aquarium is a system. Those components are various controls to keep the system in balance (e.g. temperature, Oxygen, pH, Nitrogen). Even some fish and plants play a role in recycling and cleaning, as anybody who had added a bottom feeding catfish to “control” the detritus knows. All of these are system controls, working within the system to manage and maintain it.