If you have the optional IO board support, then when you add a sensor, it will display a window asking which type of sensor to add. Otherwise it will just add an X10 type of sensor without asking you.
X10 Sensor - Any device that can send an X10 code is considered an X10 Sensor. This is usually a motion sensor, but it can also be a remote control, or even a temperature sensor.
IO Sensor - Any device that can be connected to a digital IO board and give an on or off status is considered an IO sensor. This is usually a motion sensor, but can be other devices.
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