The DC Barrel Jack provides power to the Pi and all internal circuitry. The minimum should be 12.5 Watts or 1 Amp at 12 volts. If you plan on using the external power connections or if you are using long wires, you should add the appropriate wattage or amperage needs for that equipment. The 4.5-6V two pin DC Servo power connector, should be sized appropriately for the servos running off of the servo style connectors provided with the top pins being control signals and the middle and bottom being +V and -V as typical in hobby servos. If using the Servo to LED adapter board, you will only need to connect the control and ground pins as listed in the appropriate documentation.
For the Input wires, they are optically isolated with a 2.2k ohm pull-up resistor to the Barrel Jack input. They are connected identically to the IO 8x8 and examples are in the PiFace input wiring diagrams on the ShowNodes website. They are not meant to provide power to devices but only to sense the voltage completion to ground hence the separate DC output jacks to connect active sensors like beam sensors.
For the output connections, they are a NPN Darlington transistor (also called open collector) circuit rated for 50V DC 500mA maximum current rating. They are wired identical to the IO 8x8 and the PiFace Output wiring diagrams.
Our expectation is that users will run a consistent voltage level throughout all connections and will provide all necessary power through the barrel jack. However, more advanced usages are accounted for if you need something outside the norms we have encountered.