If you have a large home with a pool or spa as well as your domestic hot water, you have a need to heat & cool at the same time. When cooling, your air conditioning outdoor unit expels heat to the outside air, when heating it expels cool air.
Energy recovery systems recover the waste heating or cooling & use it for opposite tasks, for example heating your pool & hot water when cooling your home.
These systems also monitor carbon dioxide levels within your home that build up from human activities like breathing & regulate fresh air inflow to keep carbon dioxide at safe, low levels.