The Bard HVAC MC5300-BC Lead/Lag Wall-Mount Controller is a control device designed to manage compatible Bard wall-mounted HVAC systems in lead/lag configurations. It coordinates multiple units by alternating the lead unit and bringing the lag unit into operation when additional cooling or heating capacity is required. Commonly used with Bard commercial and industrial air conditioners... https://www.partshnc.com/mc5300-bc-bard-hvac-lead-lag-wall-mount-controller-for-3-units