The Power Regulator 28V can be used in two different ways.
1. As a Power-over-Ethernet voltage regulator, it mounts on our Ethernet Shield or EtherTen to allow them to accept input voltages up to 28V via the network cable. If you only intend to pass about 10V down the Ethernet cable you don't need this module: you can simply put a pair of jumpers across the PoE header pins on the Ethernet Shield or EtherTen and let the Arduino on-board voltage regulator do all the work.
Note: This regulator module is not designed for use with commercial PoE systems such as Ethernet switches with 802.3af support. Commercial PoE switches can push up to 48V down the wire, which will overload the regulator module and force it to shut down. If you want to use the Ethernet Shield with commercial 802.3af equipment you will need to use the Power-over-Ethernet Regulator 802.3af.
If you want to push a higher voltage down the wires (perhaps you already have a 16V, 18V, or 24V power supply lying around?) the Arduino voltage regulator will get very hot and unhappy, so this regulator module takes the power you send down the cable and pre-regulates it down to 7V before passing it through to your Arduino for the final drop to 5V.
This regulator module is designed for use with home-brew PoE injectors that typically supply 16V or 24V or our 4-Port PoE Midspan Injector, not commercial PoE switches.
2. As a general purpose switchmode voltage regulator, it can be configured to provide a regulated 5V or 7V output so that you can use it directly as a high-efficiency power supply for your projects.