What is the difference?
Autofiction is a term gaining traction but seemingly with no firm definition. It doesn’t appear as a genre in Amazon for example.
It’s not a memoir and it’s not fiction. So what is it? Well, best guess from the various discussions is, it’s autobiographical, but with some variations that are fiction. For example, a sub plot that didn’t exist in the author’s life, or a character portrayed as real that the author has invented. The protagonist shares many characteristics with the author including age, occupation, where they live and so on. It might be a writer, struggling to come to terms with an event in their lives whose story depicts a writer struggling to come to terms with the same event. What makes it autofiction is when the author adds a fictional element for the purpose of the story.
Semi-autobiographical is also an autobiographical story but with some rewriting of history and extra stories woven in. A good example of this would be Trent Dalton’s, Boy Swallows Universe, which he describes as 50/50 fiction to autobiography. His view is he’s written a story that includes the things he would have liked to have done if he could have.
In this regard then, there is almost nothing to pick between the two from the available definitions, and perhaps we can all just relax and not worry about what genre we’re writing, as long as we’re enjoying the journey!