Watertight property of the input STL geometry is extremely important for meshing process. The computational mesh is created and bounded by the STLs. Any hole inside, usually at edges of neighboring parts, may cause a crash of a meshing process. It is sometimes referred to the watertight property as point-to-point correspondence of vertices. The property is visualized in Figures below.
Holes at neighboring parts
INPUT RULE 6: Each component has to be water tight.