ConfigureFPGAFromFlash does not work

I’m trying to use ConfigureFPGAFromFlash in order to reboot a XEM7310 from the system flash (the board correctly load the bitfile from flash when powered on). The fucntion requires a parameter ConfigIndex that is not clearly explained in the documentation. I tried putting several reasonable values (0, 1, 16), but all I obtain is a reset of the FPGA without reloading the firmware from the Flash. If Instead I use ConfigureFPGA and provide a valid firmware file as a parameter, the board reboot and reconfigure correctly. Someone can help me?