(User’s Manual: D300S page 96; D300 page 96)
ISO sensitivity settings are provided to give you control over the light sensitivity of the imaging sensor, including whether you manually control it or the camera sets it automatically.
An ISO sensitivity number, such as 200 or 3200, is an agreed-upon sensitivity for the image-capturing sensor. Virtually everywhere one goes in the world, all camera ISO numbers will mean the same thing. With that fact being established, camera bodies and lenses can be designed to take advantage of the ISO sensitivity ranges they will have to deal with. Standards are good!
In the D300(S), the ISO numbers are sensitivity equivalents. To make it very simple, ISO sensitivity is the digital equivalent ...