Thanks Rolf, this does show it more clearly than what I’d written!
One thing I found when writing this component (and you might need to watch out for if scripting something similar) was that if you want to distinguish between a ‘mountain’ and a ‘valley’ fold on the surface, you need to know which of the 2 faces sharing an edge is on the left, and which is on the right (when looking along the direction of the edge, and with ‘up’ being outside the surface). Without this you can get the angle between the face normals, but not know whether they are folding towards or away from each other.
There is an overload that Steve added to GetConnectedFaces that helps with this:
https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_Collections_MeshTopologyEdgeList_GetConnectedFaces_1.htm
That boolean lets you know which side is which.