The external voltage level shifter is for interfacing to SD cards. The SD I/O signals are routed through one of the two PMC MIO banks (0 or 1). The I/O voltage for the entire 26 pin MIO bank is from one set of power pins. They are usually supplied with 1.8 or 2.5V. For SD 3.0 boot and other applications, an external voltage level shifter is needed to enable the controller to initially interface at 3.3V at the card and then a lower voltage for high-speed transfers.
The wiring diagram for an SD card connected to a voltage level shifter is shown in the following figure. This example shows the PMC MIO bank 0 and is powered by the VCCO_500 power pins. PMC MIO bank 1 is powered by the VCCO_501 power pins.