Solids are closed volume of surfaces that can take any shape. Solids are three-dimensional entities that can be used in automatic tetra and solid meshing.

The surfaces defining a solid can belong to multiple component collectors. The display of a solid and its bounding surfaces are controlled only by the component collector to which the solid belongs.
Bounding Surface
A bounding surface defines the outer boundary of a single solid.
Bounding surfaces are shaded green by default.
A bounding surface is unique and is not shared with any other solid. A single solid volume is defined entirely by bounding surfaces.
Fin Surface
A fin surface has the same solid on all sides, that is, it acts as a fin inside of a single solid.
Fin surfaces are shaded red by default.
A fin surface can be created when manually merging solids or when creating solids with internal fin surfaces.
Full Partition Surface
A full partition surface defines a shared boundary between one or more solids.
Full partition surfaces are shaded yellow by default.
A full partition surface can be created when splitting a solid or when using Boolean operations to join multiple solids at shared or intersecting locations.

Figure 1.