Besides a data link, USB provides 5 +/- 0.25 volts to power an otherwise unpowered device or charge a battery in a self powered device. A USB controller or hub is required to power one unit load (100 mA, a low power load). It can optionally power up to 5 unit loads (500 mA, a high power load). The spec is silent about intermediate loads, so I assume that the sources deal with just the two cases. Either a load is low power (like a keyboard or a mouse) or high power (like an iPod or PDA).