[Radio Wrangler]

It's a high voltage range extender for a voltmeter.

Charging a larger capacitor near ground via a smaller capacitor from the unknown voltage puts a calculable fraction of the unknown voltage on the larger capacitor.

Used because it's a lot more difficult to make reliable high voltage resistors than it is to make high voltage capacitors. Used for measuring high voltage AC, can be used on dc as a one-shot measurement but that's difficult.

The capacitance of whatever you feed it to and the cable are in parallel with the output capacitor and affect the accuracy, hence the design value is carefully labelled.

David