It doesn't change fuel pressure, it adds/subtracts to the fuel calculation, to attempt to compensate when the fuel pressure is not maintaining the proper differential pressure.
If your fuel pressure is set to 43psi (line off), then normally you should always have the 43psi differential regardless of boost or vacuum. If it starts to fall, for example, the fuel pressure correction will increase duty cycle. If you don't have enough fuel volume (failing pump), it can't really fix that though.