Transfer relays are used for transfering power source between two independent sources in order to provide continuous energy flow for critical loads.
Generally, independent sources are uninterruptable power source (UPS) and mains power. In normal operation, critical loads are supplied from the mains power as the primary. When there is a problem with the primary source like low/high voltage or blackout, transfer relays detect the failure and automatically transfers the source to a secondary supply.
Automatic transfer relays are often used in suppling medical loads in hospitals especially the equipment that are used in operating theatres and intensive care rooms but could be used anywhere that constant power is vital.