Returns the thickness of the planar solid entity (DXF 39) in drawing units. Thickness is the extrusion length along the normal. A positive value defines the thickness to be drawn along the normal direction. A negative value defines the thickness to be drawn in the opposite direction from the normal. A zero value defines a planar solid without thickness.

Example of Working with the Planar Solid Object 

Specific Planar Solid Properties 

OdDbSolid::setThickness() method