What writing characteristic determines that a biography is autoboigraphy?

The key writing characteristic that determines a biography as an autobiography is the author's identity.

Here's the breakdown:

* Biography: Written by someone other than the subject of the book. It tells the story of another person's life.

* Autobiography: Written by the subject of the book themselves. It tells the story of their own life.

Therefore, the author's perspective is the deciding factor. If the author is also the subject of the book, it's an autobiography.