Any module should be able to extend the system by providing additional thingies that can be chosen from, but at any given time, only the actually chosen one(s) should be invoked. For example, the Image module lets you create image styles by choosing from a set of image effects (scale, crop, rotate, desaturate, etc.). Any module can add more effects to the system. However, a particular image style is configured as a particular sequence of effects, and when it's time to generate an image of that style, the Image module wants to invoke just that particular sequence of effects.