The textbook actually does a poor job of defining this most important stage. The reason for this is because it is difficult to map out accurately what happens in someone's mind before writing. What can be described more easily is the analysis a writer makes automatically when handed a writing assignment. The writer wants to know a few things. First, the writer looks at what the topic is or ought to be and how to approach the topic.
People then try to draw on their personal experience to fill in the topic.
For instance, given an assignment to define the complex idea of intelligence, the writer might find in his or her personal experience that the SAT test the writer took in high school put him or her in a sub-par level for future academic success, which may or may not have affected the writer's life in many different ways. He or she may then use this experience to refute the idea that there is one right way to define intelligence. However, in the planning stage people try answering these basic questions. And the more precisely one can find answers to these questions, the more precise and easier to understand the writing will be. What many inexperienced writers overlook is this stage's importance, as it sets the foundation for everything to follow when putting words on paper.