Think of a flow switch as like the indacators in a car, when you have a oil light come on in you car it is because a "switch" is closed when the pressure is lower then a preset level but if your car has a gauge then the switch is replaced with a sending unit, which is a varible resistor that will move a guage, the later will have more wires to send this info to the gauge, how a flow meter sending unit is mounted to a pipe or other means of fluid transmission method will depend upon the system you are working with, but there can be more wires since there is more info to be transmitted.