What the different between a reference sensor and crankshaft sensor

Engine Management Computer system uses two crankshaft sensors. One (Reference Sensor) determines the #1 piston (front of the engine) position relative to Top Dead Center (TDC). The TDC is the pistons highest position on the compression stroke. The second sensor (Speed Sensors) counts the teeth on the flywheel and provides this signal in the form of pulses to the Engine Management Computer for engine speed in RPM (Revolutions Per Minute). Using the combination of the two signals, the DME can determine where the engine position is at any point in its cycle to apply the correct fuel injector cycling and correct ignition firing. The speed sensor's signal is also used as a safety feature by the DME (after initial cranking is complete) to shut down the fuel pump if engine speed falls below 300 RPM. This information is from my Ford Shop Service Manual, revised August 2016.