Solids are geometries that have a volume. Solids must be 3D (x,y,z).
The boundaries of Solids are surfaces. They have outer boundaries and some solids may have an arbitrary number of inner boundaries. The 3D holes that the inner boundaries define are called "voids".
Solids that contain surfaces infer their orientation from the arrangement of the underlying boundary surfaces. In FME, solids have three possible orientations, where the surface normals of each surface that comprise the solid either:
The first case is most typical; the front faces of the surfaces are seen on the outside of the solid.
The second case is equivalent to turning the solid inside-out, such that what is visible is the back of each surface. This is useful in some cases, and is expected for solids that are intended to be voids within a larger solid.
The third case may arise because FME does not restrict solids to have coherent surface normals between the solid boundaries. If required, transformers may be used to alter or repair Solid orientations.