On a digital controller, the setpoint is the number you enter, for example 3 °C for a reach-in chiller or −20 °C for a freezer. Depending on how the controller is configured, the compressor usually stops at the setpoint and restarts when the cabinet warms by the differential, so the average cabinet temperature sits slightly above the setpoint.
Choosing the setpoint is a balance. For chilled food, common guidance is to keep it at or below 5 °C, so many kitchens set 2 to 4 °C to allow for door openings and the warmest shelf. Frozen storage is commonly at or below −18 °C. Setting far colder than needed wastes energy, can freeze delicate produce in a chiller and increases frost.
The setpoint is the temperature at the probe, not necessarily at every point in the cabinet. A probe near the evaporator outlet reads colder than food near the door. Check product or simulated-product temperatures on different shelves when you first commission a cabinet, then adjust the setpoint if needed.