Editing Solids and Surfaces - Autodesk AutoCAD 2013 Practical 3D Drafting and Design - page 225

• seParate solids : If one solid has separated volumes, this option allows
you to separate these volumes into independent solids. It prompts only
for a solid selection.
• Shell : This option allows you to create a hollow solid with a constant wall
thickness, eventually open through one or more faces. We can only apply
this option once to a solid. After the solid selection, it prompts for the se-
lection of faces to remove, thus creating an open thin-wall solid, and the
shell thickness. A positive thickness creates the shell on the inside, while
a negative thickness creates the shell on the outside.
Note
We must be careful with this option. Immediately after the solid se-
lection, it prompts for face removal. If we press Enter , we remove no
faces.