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.