My question is whether there is any real difference between the functioning of a diverter and rough-in valve piece. As I understand it, a diverter is the piece which "diverts" the water between one and three different water outlets. If that's so, then doesn't a rough-in valve function similarly? I've heard the rough-in valve described as the "engine" of a bath or shower. Some clarification between these two terms would help a lot. Thanks!