When you create elements using tools in the Primitive Solids toolbox, the Capped option in the Geometry section of element Properties specifies whether the 3D element is either of the following:

• Capped = Yes — Element is a Solid and is capped on its ends; encloses a volume.
• Capped = No — Element is a Surface and is not capped on its ends; does not enclose volume.

Where you have a 3D object that is a surface, you can convert it to a solid, and vice-versa.