Chan,

Even in the digital world there is a native rating rating for any given camera, all the other settings are essentially the processing being adapted. People tend to think of the setting as a real change but it is not, the sensor does not change.

It is like switching channels on your radio, some stations are louder than others, we use the volume control to adjust. Digital shooters use the ISO knob to similarly dial up or down the gain.

Our analog equivalent is our enlarger exposure settings.