Let's step back and consider a simple definition of a 'solid'... A "container" - group or component - is considered to be a 'manifold solid' if it just contains 'faces and their edges' [note that 'guide geometry' is ignored]; but nested group or component-instances within it will mean failure. The "container's" edges must all have faces... If any edge has no faces then the "container" is not a solid. If any edge has only one face then it's forming a 'flap'... so then the "container" is not a solid. If any edge has more than two faces it's forming an ambiguous form - an internal 'partition' - so then the "container" is not a solid. So a 'manifold solid' contains edges that all have two faces - i.e. no more than two faces, no fewer than two faces... If it doesn't then it ain't a 'solid'