Definitions:
A manifold triangle is one that is connected to others on all three sides.
A manifold object is a polygon object whose triangles are connected on all three sides (except for boundary edges).
A closed manifold object is a manifold object that encloses volume. Some applications like rapid prototyping require that a polygon object be closed-manifold.
An open manifold object is a manifold object that does not enclose volume.
In some applications like rapid prototyping, a polygon object must be closed-manifold.
If the point cloud data represents a closed object, Points > Wrap often creates a closed-manifold object. If the scanned data is noisy, however, Wrap can create an object that is open-manifold or not manifold at all.
Polygons > Make Manifold has two variants.
Make Manifold > Closed - deletes non-manifold triangles from an otherwise closed manifold (volume-enclosing) object. On an open-manifold object, all triangles would be considered offensive and the entire object would be deleted.
Make Manifold > Open - deletes non-manifold
triangles from an otherwise open manifold object (such as a a sheet metal
part).