The following is, imho, the best general workflow for adjusting exposure.
Use the Exposure Slider to correct for gross differences between the recorded exposure and the desired exposure. Use the Black Point Slider to correct for gross underexposure. Use the Recovery Slider to correct for small areas over-exposed. (The Modifier Key Color Overlays are helpful.)
After thus dynamically setting the _limits_ of the exposure, use either the Brightness Slider or the Curves Brick or the Levels Brick to adjust the distribution of the luminance values inside those limits.
Then, as needed, use the Highlights Slider to lower the luminance of the lightest lights in your luminance range, and the (senselessly misnamed*) Shadows Slider to increase the luminance of the darkest darks in your luminance range.
The guide to the colors used by the Modifier Key Color Overlays is here in the User Manual.
*One of the first things beginning painters are taught is that "shadow" and "dark" describe importantly different phenomena. Aperture has no idea what a shadow is.
Thanks very much. You have helped me understand the process and I am now better able to tackle the problem.I appreciate the time you have taken to reply to my query. I recognise that photo-editing cannot be definitive, as a good photo, is in the eye of the viewer and is therefore, subjective, but I now have some good guidlines to work to, in using Aperture.