The diagnosis answers one question: what is going on here. Not the symptom, the challenge. Rumelt spends most of his time here, because this is the part organizations skip. Skip it and everything downstream goes wrong without anyone noticing.
The guiding policy comes next. It is the approach you are taking, and Rumelt calls it the core of a strategy. It says how you will deal with the situation you just named. The word most people reach for here is priority, and they use it wrong.
He gets the image from air traffic control. A tower gives a pilot priority over the next three planes on the runway. It means first, in a queue, right now. A list of 17 things is not a queue. Rumelt had a client bring him that list, and he calls it the opposite of a policy. It held everything the company hoped would happen next year. Market share in China. Lower costs. Better safety. All at once.
Then comes the part he says is the easiest to leave out, because people like to think of strategy as a high-level conceptual thing. Coherent action. The actual moves, and they have to point the same way. A guiding policy with no action attached is a sentiment, and actions that fight each other cancel out.