Nonmanifold Parts
Condition
Nonmanifold is a term used to describe parts that are physically impossible to manufacture. A few common examples include two volumes intersecting at a single point or edge, or boxes with holes trimmed perfectly at their edges.
Cause
These parts can be accidentally created from time to time while performing solid editing operations, resulting in an error message when you run an optimization.
Remedy
If you receive this error message, it will include a list of parts that
you need to modify in order to correct the problem.