Clusters in my code? Oh, you mean the ‘Tree/List Viewer’ tool? That’s a general purpose utility for visually examining data trees of geometry. You don’t have to understand it to use it, though you are welcome to examine their innards.
For the purposes of this thread, you can treat it as a black box. Sometimes it’s necessary to disable other previews if they override what it is trying to show.
Oh, you’re that guy? 
Not sure I did you any favors with my speculations in that thread.
I’m learning about meshes myself in this thread. I never knew that, if not “welded”, they can have an internal data structure similar to polysurfaces? Because from what I can tell so far, there is no possible way to get at that structure through standard components?
This exercise to identify “naked” and “clothed” vertices has been illuminating. Naked vertices by edge using Mesh Edges (MEdges) makes sense but from what I can tell, getting clothed vertices by edge depends on knowing the interior edges from the brep represented by the mesh, if there is one. It must be possible somehow without a brep(?) but I haven’t seen it yet.