When thinking of story in terms of Dramatica, it can be helpful to remember that characters are not real people—they’re aspects of a single human mind processing a single problem.
With this in mind, Backstory is an analogy for the justification process. In other words, Backstory explains why a Main Character Problem is a Problem.
I find it helpful to think of the justification process of Backstory as the “opposite” to the Main Character Resolve for the narrative in question.
For instance, if this is a Changed Character then the Backstory would explore the Steadfast narrative that led to the justification now seen as a Problem. Same with Steadfast Characters—Backstory is seen as the “Changed” story before.
In truth, narratives are a constant wave of Changed to Steadfast to Changed to Steadfast and so on for all eternity—for a SINGLE MIND. A narrative—or story—is simply an arbitrary moment in time when the Author sets out to determine beginning and end for that one mind’s consideration.