The purpose of capacitor discharge units is to store up electricity in a capacitor. The electrical charge is released, on throwing a switch, as a burst of a much larger current than a transformer can supply.
The CDU can be supplied with either an AC or DC voltage up to 24v. Operating from 24 volts typically gives 4 times the power. In most cases a voltage of 16v AC is  ideal to power a CDU. The output of a CDU is always DC. The Standard unit will power up to 4 sets of points simultaneously.