I don't understand Jim's theory/model, its quite complex, something an engineer would devise.
But I see the benefits of separating the Protagonist and Main character--at times.
When the protagonist and main character are allowed to be two different characters, I think we can be more flexible and creative when developing a story.
The protag and MC can be the same character. It's our story, our choice.
No longer must our main character pursue a single external goal that affects everyone. Our main character, the character who our story is about, the character we will learn the most about, can pursue a variety of internal and external goals as the story progresses.
Meanwhile, another character, call him the protagonist, pursues the story goal and is instrumental in driving the complete story forward as our main character faces all kinds of obstacles not even related to the story goal.
Let's call these characters, what they are.
Character 1. Character Who The Story Is About. (Once called the Main Character)
Character A. Character Who Pursues The Goal That Shows Up In Our Logline (Once called the Protagonist)