This Kilovolt DC Power Supply is used for electron tubes and electrostatics.
The high voltage section is by design “floating” relative to ground. This means that either the black (negative) terminal or the red (positive) terminal may be connected to ground to give a voltage range of 0 to +6 kV, resp. 0 to -6 kV relative to ground.
The output is well regulated, and the current is limited for safety.
(The maximum short circuit current is 2 mA).