Hot tubs typically use either a flow switch or pressure switch. Jacuzzi Hot Tubs tend to use a flow switch. In either case, the purpose of the switch is to tell the board that enough water is moving pass the heater element. If not, the topside will give you a FL1 error. If the switch itself has failed you get an FL2 error.
Flow switches are directional. You must install the new with the arrows on the switch and housing pointed in the water's direction.
Pressure switches and not directional. Pressure switches have a "sweet spot" where when the pump is off, the switch is open and when the pump is on, the switch is closed. You may have to adjust a pressure switch if you start getting either a FL2 or FL1 error.