The iLS120-ON directional relay is useful for adding equipment to augment a process like heating or cooling. It allows you to add additional equipment exceeding the controller load capacity: the equipment is simply activated when the current is active at the controller output.
The iGS-DSC is an interface relay low voltage circuit used to control high power equipment. It is usually used as an interface to a climate controller for control conventional low voltage circuit for heating, air conditioning or anything else.
The iLS120-OFF reverse relay avoids conflicting operations: for example, it is useful to inhibit CO2 enrichment, a dehumidifier and/or other equipment on another circuit when ventilation equipment is running.