This does not appear to me to be a conventional metering circuit; it looks instead like a monitor for device voltage and current. U2 likely provides a reference voltage (it's a 3-terminal device, though, but is not used in any typical configuration).

I suspect that the two variable resistors are used to calibrate the circuit, and the zener diode (DZ1) provides overvoltage protection on RA0.

If you seek only to measure current, there are simpler circuits.