Couverture is essentially made of cocoa mass (except in white couverture), cocoa butter, sugar and dairy products (except in plain couverture). Pure cocoa mass and cocoa butter can then be used to alter the characteristics of a basic couverture or a filling.
For instance, if a chocolate with the taste you're looking for is just a little too viscous for a particular enrobing line, it is possible to increase its fluidity by adding a little cocoa butter. Cocoa mass would generally be used to enhance the bitterness of a chocolate, although this would also have the added effect of decreasing its fluidity.
Please note that cocoa powders will change colour depending on the application to which the powder is applied.