how to understand these concepts such as "dead verts,dead edges,dead faces..."

About Editable_Poly Dead Sub-Object Elements:
Editable polys can have "dead" vertices, edges, and faces.

These are Sub-Object elements that have been deleted from the poly, but have not yet been removed from the corresponding lists of Sub-Object elements.

Sub-Object elements may be marked as dead in the process of performing certain operations on the poly, but the Sub-Object element lists are normally collapsed (that is, all dead Sub-Object elements removed) at the end of the operation or at file I/O operations. Thus, normally there should not be any dead Sub-Object elements present in the poly.

If you flag a Sub-Object element as dead using one of the polyOp methods, you should also call polyOp.collapseDeadStructs() to collapse the Sub-Object element lists.

Other than methods that get/set Sub-Object element flags, dead Sub-Object elements are normally ignored by the polyOp methods. If a method returns a value other than OK, takes a single Sub-Object element index, and that SO element is dead, a value of undefined is normally returned.

http://docs.autodesk.com/3DSMAX/16/ENU/MAXScript-Help/index.html?url=fil…