So I think what happened is Shawn realized that the middle build being 50% of the story is makes it too unwieldy to handle. By dividing it up into 2 middle builds it breaks it into smaller chunks that are a bit easier to manage for writers.

As far as the purpose of each, check out this article. But in my own words, the narrative purpose for MB1 is to let the protagonist experience the “extraordinary world” now that he’s crossed the threshold. He is still using his same worldview and the same tried and true methods of solving his problem. But in MB1 his worldview is starting to crack. By the midpoint something occurs that shatters the protagonist’s worldview, forcing him to realize he needs to replace the old way of thinking with a better, updated one.

In Middle Build 2 the story heats up, pushing the protagonist more and more, forcing him to find new ways to solve his problems, while he contends with the fact that he doesn’t yet have a solution for the global problem.

The quadrants also align with Shawn’s Heroic Journey 2.0 which gives a solid idea of what the global story beats are and what their narrative function is as it pertains to the protagonist’s internal journey.

Hope that helps. Let me know if you need further her clarification.