This might seem a strange place to start, but from an editor’s perspective, I would say that it’s the most important thing that you need to know to get started on your writing project.
It’s also deceptively simple. If you’re a scholarly writer, you might say, “That’s easy. It’s for people in my field of study.”
If you’re working on a corporate or institutional report, your answer might be, “It’s for the people who told me to write it.”
But even those apparently straightforward answers are actually not a simple as they seem.
So you’re writing for scholars in your field. What exactly does that mean?
For people who have exactly the same level of knowledge about the subject as you do?
For people who work in your area of specialization or everyone in your general field? People in allied fields?
Could your writing ever be used for entry-level students? Is it possible your eventual publisher might try to widen the audience for the ideas you propose?
And if you’re working on an institutional or corporate report, who’s really going to see it? Current staff? Potential donors? Foundations who might be considering giving a grant to your organization? People you want to recruit as volunteers? Members of in the community where you are located?
See? Not as simple as it sounds.