The speed sensor converts the water pressure from the lower units pitot tube to a digital signal inside the engines ECU.  The ECU then displays the speedon the speedometer.  The kits contains all of the components necessary to connect the sensor to the pitot hose coming from the lower unit and one kit fits all.  V8 outborads do not have speedo tube required for connection to this sensor (Replacement sensor: 6AW-8366B-01-00)