When you’re planning to write, but before you’re actually writing, you create an outline. Unfortunately, most outlines are worthless. You need a better outline: a fat outline.
Outlines are helpful for mapping out the structure of a long piece of writing — anything more than 1,000 words (a couple of pages). An outline ought to help the people you’re working with — your boss, your clients, your editor — to understand what you’re going to write. It should also force you, the writer, to think clearly about content.
The problem is, traditional outlines don’t do this very well.
Josh Bernoff says fat is good – that is when you're writing an outline and intending to share it with editors, your boss or others.