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.

When the create mode is set to FE geometry, the output is FE geometry with element tessellation.