In the iMetos 3.3 board there are several test points for convenience during service, they let the user access different voltages with no need to unplug or removing anything.
GND: common ground
BAT: battery voltage
SOL: solar panel voltage
VSUP: internal voltage supply 6V
V31: internal voltage supply 3.1V (processor, hygroclip sensors...)
V54: internal voltage supply 5.4V (sensor inputs...)
Example: if you wish to measure battery voltage, you can measure it with Voltmeter on GND and BAT test points.