The mission statement
Any system you build, no matter how complicated, has a fundamental purposethe business that its in, the basic need that it satisfies. If you can look past the user interface, the hardware- or system-specific details, the coding algorithms and the efficiency problems, you will eventually find the core of its beingsimple and straightforward. Like the so-called high concept from a Hollywood movie, you can describe it in one or two sentences. This pure description is the starting point.
The high concept is quite important because it sets the tone for your project; its a mission statement. You wont necessarily get it right the first time (you may be in a later phase of the project before it becomes completely clear), but keep trying until it feels right. For example, in an air-traffic control system you may start out with a high concept focused on the system that youre building: The tower program keeps track of the aircraft. But consider what happens when you shrink the system to a very small airfield; perhaps theres only a human controller, or none at all. A more useful model wont concern the solution youre creating as much as it describes the problem: Aircraft arrive, unload, service and reload, then depart.