Occasion is why the author started writing it in the first place. Like, I wanted to write about leaves today because I was so busy crunching them. Olaudah Equiano wrote about slavery because people didn't know about it, and as a slave he had a unique perspective. Purpose is why an author wrote about it, why it was important to him. I think you can figure that out for most of these authors. Audience is who the author wrote it for. Equiano wrote for the upperclass, people who thought slavery was harmless and humane.
