Scanner takes a 0-5V input voltage and turns it into 4 0-5V overlapping triangular output voltages. A classic "weirder to type about than in practice" module. Scanner is useful for driving VCAs (for a 2-4 way crossfade), waveshaping (any input wave comes out a triangle), and probably lots more!
The individual outputs are not truly named, but arranged in a square, progressing clockwise. If you call the lower left "A" and the lower right "D", this diagram shows the input / output relationship: