What a non-manifold STL report tells you.
A mesh is a collection of triangles. How those triangles meet matters as much as how they look.
Open edges
An edge with one incident triangle lies on a surface boundary. It may outline an accidental hole, an intentional opening, or a sheet with no thickness. Closing every boundary can destroy the design. MeshMend displays these edges and only attempts simple planar caps when you enable hole filling.
Non-manifold edges
An edge shared by more than two unique, non-degenerate triangles branches in a way that a simple solid surface cannot. Removing duplicate faces can resolve some raw mesh problems. Arbitrary branching needs a modeling decision; this tool leaves it reported rather than choosing which surface to delete.
Non-manifold vertices
Two otherwise closed shells can touch at a single vertex. Every edge may still have exactly two faces, yet the vertex has disconnected surface neighborhoods. That is why this tool checks vertex fans as well as edge counts.
Winding conflicts
Adjacent triangles on an oriented surface traverse their shared edge in opposite directions. A conflict often comes from a reversed triangle. The repair pass propagates consistent winding and regenerates normals on export. Nested shells are more subtle: inward-facing geometry can describe a cavity.
When to use a desktop editor
Use a mesh editor or your original CAD model when the report still shows branching surfaces, the intended solid is ambiguous, or the repair would close an opening you need. Re-exporting from the source CAD object is often clearer than reconstructing a heavily damaged triangle surface.
Self-intersections are a separate geometric problem and are not checked here. Zero reported topology defects does not certify a printable solid.