If current flows into the positive voltage terminal, the power supply is sinking current and is acting like an electronic load – it is absorbing and dissipating power instead of sourcing power. Most power supplies do not do this although many Agilent power supplies can sink some current to quickly pull down their output voltage when needed – this is known as a down-programmer capability – see this post for more info: http://powersupplyblog.tm.agilent.com/2012/03/if-you-need-fast-rise-and-fall-times.html.
As mentioned earlier, some power supplies are uni-polar (produce only a single polarity output voltage), but can source and sink current. These power supplies can operate in quadrants 1 and 2 and can therefore be called two-quadrant supplies. In quadrant 1, the power supply would be sourcing power with current flowing out of the more positive voltage terminal. In quadrant 2, the power supply would be consuming power (sinking current) with current flowing into the more positive voltage terminal.
Some power supplies can provide positive or negative voltages across their output terminals without having to switch the external wiring to the terminals. These supplies can typically operate in all four quadrants and are therefore known as four-quadrant power supplies. Another name for these is bipolar since they are able to produce either positive or negative voltage on their output terminals. In quadrants 1 and 3, a bipolar supply is sourcing power: current flows out of the more positive voltage terminal. In quadrants 2 and 4, a bipolar supply is consuming power: current flows into the more positive voltage terminal. See Figure 3.
Agilent’s N6784A is an example of a bipolar power supply. It can source or sink current and the output voltage across its output terminals can be set positive or negative. It is a 20 W Source/Measure Unit (SMU) with multiple output ranges. See Figure 4 for the output characteristic of the N6784A.
To summarize, a bipolar or four-quadrant power supply is a supply that can provide positive or negative output voltage, and can source or sink current. It can operate in any of the four quadrants of the voltage-current coordinate system.