What is a non-manifold mesh and how to fix it - Sinestesia
modeling - What is non-manifold geometry? - Blender Stack Exchange
How do I fix these manifold issues? And, should I have to "fix" the model in Simplify3d every time, or can I address that in Blender? (Blender -> Simplify3d) : r/3Dprinting
Types of non-manifold geometry | CGTyphoon
Tips for a successful manifold 3D model – Help Center
Manifold Meshes · rlguy/Blender-FLIP-Fluids Wiki · GitHub
Non manifold selection disappearing from menus - Usability - Developer Forum