OK - Thank you to Geoff; Forseti; ap4a and Chris-p and all others that agreed or disagreed
So to test my understanding -
If I take a photograph at (say) ISO 400; F5.6; at 1/250th second and I look at the histogram, then I can change any of them to a maximum of +/- "clicks", giving a spread of 5 or 6. ideally, and without killing the highlights, that will move the histogram to the right, to preserve as much information / retrievable image as possible.
The decisions I make about which to move include speed of subject; depth of field; noise; etc as normal
Finally (Sorry) If I include compensation, and am on "P", does the camera decide which to change? I assume it has criteria programmed , with ISO last to be moved.
I clearly need to get out more - with the camera.
Bob