Page 37 tells it all: "Auto ISO sensitivity control can be enabled in modes P, S, A, and M using the Auto ISO
sensitivity control option in the ISO sensitivity settings menu".
So, if turned on, camera changes the basic ISO value (user defined) in order to take care about the lowest allowed exposure time (user defined) and to meet the limitations defined by other elements of the exposure. If turned off, camera keeps the predefined ISO value on the "no matter what" basis, and will use any of the available combinations of exposure time and values of aperture to maintain the correct exposure...