PS2 is a power supply provided by the NT1. Not all NT1s can do this as standard. For instance, in the UK this is a separate, chargeable feature.

PS3 is a mechanism through which one device can supply power to other devices. This is very rarely implemented.

There is another power supply known as PS1. This is also known as the "phantom" power supply. It is found in the voltage difference between the transmit and receive data pairs. This derives from the network itself and is normally used for emergency situations where the user's local power supply has failed. Contrast this with PS2 where the NT1 provides this power from its own mains plug.