MECE stands for Mutually Exclusive, Collectively Exhaustive. This means that collectively the ideas in the storyline should exhaust the entire relevant universe of ideas set up by the overarching answer, and that those sub-ideas should mutually exclude each other. In other words, MECE could be described as “no gaps, no overlaps” or NONG. No ideas should overlap each other, and there should be no gaps between the ideas and no gaps within the overarching story.
Identifying whether a set of ideas passes the MECE / NONG test is difficult, and ordering your ideas visually into a hierarchy seems to help people ‘see' the gaps and overlaps better than if working within an actual document.