The input is marked 0-300V on the PCB. That rating is commensurate with the power capability of the two 4.7kΩ 2W and single 10kΩ 2W resistors shown. It goes to a full-wave bridge rectifier and some passive low pass filtering, and thence to an optoisolator.

On the other side there is a mechanical trimpot and some more filtering and it goes into the ADC input of an MCU of some kind (likely a Microchip PIC type, based on the package and pinout).

For the optoisolator input LED to conduct it needs around 1V (about 1.2V at a reasonable current). The voltage at the input would have to be higher due to the voltage drop in the bridge rectifier etc. The markings may refer to the voltage under some particular conditions for troubleshooting or setup purposes.

Note that there are some differences between the board image and the board layout.

The input is marked 0-300V on the PCB. That rating is commensurate with the power capability of the two 4.7kΩ 2W and single 10kΩ 2W resistors shown. It goes to a full-wave bridge rectifier and some passive low pass filtering, and thence to an optoisolator.

On the other side there is a mechanical trimpot and some more filtering and it goes into the ADC input of an MCU of some kind (likely a Microchip PIC type, based on the package and pinout).

For the optoisolator input LED to conduct it needs around 1V (about 1.2V at a reasonable current). The voltage at the input would have to be higher due to the voltage drop in the bridge rectifier etc. The markings may refer to the voltage under some particular conditions for troubleshooting or setup purposes.

Note that there are some differences between the board image and the board layout.

I would guess they are using an optocoupler designed primarily for digital use in an analog mode. There's nothing wrong per se with this, however stability won't be great and there will be a huge manufacturing tolerance (probably why it needs the potentiometer).

The input is marked 0-300V on the PCB. It goes to a full-wave bridge rectifier and some passive low pass filtering, and thence to an optoisolator.

On the other side there is a mechanical trimpot and some more filtering and it goes into the ADC input of an MCU of some kind (likely a Microchip PIC type, based on the package and pinout).

For the optoisolator input LED to conduct it needs around 1V (about 1.2V at a reasonable current). The markings may refer to the voltage under some particular conditions for troubleshooting or setup purposes.

Note that there are some differences between the board image and the board layout.

The input is marked 0-300V on the PCB. That rating is commensurate with the power capability of the two 4.7kΩ 2W and single 10kΩ 2W resistors shown. It goes to a full-wave bridge rectifier and some passive low pass filtering, and thence to an optoisolator.

On the other side there is a mechanical trimpot and some more filtering and it goes into the ADC input of an MCU of some kind (likely a Microchip PIC type, based on the package and pinout).

For the optoisolator input LED to conduct it needs around 1V (about 1.2V at a reasonable current). The voltage at the input would have to be higher due to the voltage drop in the bridge rectifier etc. The markings may refer to the voltage under some particular conditions for troubleshooting or setup purposes.

Note that there are some differences between the board image and the board layout.