Switching power supply -V
The V- is the more-negative terminal of the DC output.

Line (L) and Neutral (N) are the AC mains inputs. The ground (⏚) symbol is for protective earth.

The DC outputs are the +V and -V, where V- is the more negative connection.

The DC outputs may be floating, relative to the protective earth input. This would allow you to connect two of these power supplies in series, and ground the midpoint, which would produce a bipolar, protective-earth referenced power supply.

As to where you would use it, you would use it any time you use the power supply. You have to use it to use the power supply. It's where you connect for one terminal of the DC output.