When there's no input source connected, the set voltage (on the back, 5/10V) is normalized to the input, this way you can use it to generate CV in itself. The LED corresponds to the activity of the output, green is positive, red is negative voltage. If you switch to BIPOLAR mode, 0V will be in the center of the fader, down is negative voltage, up is positive. When you connect an input source you can attenuate it with the fader.
FADE gets more interesting if you LINK more units, this way you can mix signals together, or offset them easily. To mix signals, insert a cable in the output of only the last unit you want in the chain, this breaks the normalization and the signal won't appear further down the chain. You can set up a master volume by connecting the output of the module before the last one to the last one's, so that the last fader attenuates the mixed signal. If you connect an input source in unit 1, and no input to unit 2, you can use unit 2 as an offset for the signal, great for LFOs, envelopes, etc.