Soup is a form, often a side dish, or a single focus meal. It is an important form when beginning a heavy meal because it warms the digestive tract, readying it for the meal. Hence may be called an appetizer.
The style of soup, like sauce, is defined by the method of preparation. In theory, soup is the boiling method, where everything in the massive amount of cooking liquid becomes the soup and is eaten. Unlike the boiling method for pasta where the liquid is thrown out.