The regular power supply mode is considered a first-quadrant operation where both the voltage and current are positive. A source meter or an SMU behaves as follows in each of the quadrants:
In the first quadrant, the source meter is a source with a positive voltage across it and is supplying a positive current from its output terminal.
In the second quadrant, the source meter acts as a load with a positive voltage across it. It sinks or receives current from the DUT at its output terminal.
In the third quadrant, the source meter is again a source supplying current, but its polarity is the opposite of conventional polarity and hence negative. That means the return terminal is set at a higher voltage than the output terminal but without changing the wiring to the DUT.
In the fourth quadrant, the SMU again acts like a load. The voltage is negative, which means the return terminal is at a higher potential than the output terminal. However, the current flows conventionally.