If you reset it from the HTTP(s) GUI or the serial CLI you will be given the option for to either "Reset Configuration" or "Factory Reset Device". Reset Configuration does exactly as it sounds, Factory Reset is the one that sends it back to the exact state you took it out of the box in.
That said I think that you can do either via the button on the back depending on time pressed (<10 sec or >= 10 seconds seems logical), however I am not 100% certain as I don't use the reset button, because I don't get the instant feedback I get via serial.