A Copper Brush should be used before waxing (to clean ski base and open it for waxing) and after scraping to expose the bulk of the base structure.
A Horsehair Brush should be used when waxing with a cold hard wax such as Blue. It should follow brushing with the Copper Brush.
The Nylon Polishing Brush should be used when waxing with the medium and soft waxes such as Red and Yellow. It is a finishing brush that should follow brushing with the Copper Brush. This is also the best brush for finishing fluorocarbons.
The Nylon general purpose brush is a great brush if you only have one brush.