The KFUH-2S1 2-stage control relay is required when you have a factory-installed KFUH-2TS 2-stage thermostat. The KFUH-2S1 2-stage control relay is for use with single phase models. A control relay is factory installed and must be ordered at the same time as KFUH-2TS.
The KFUH-2S3 is a 2 stage control for three phase models when using the factory installed KFUH-2TS thermostat.