Which option is NOT typically part of the Shell command in 3D modeling?

In the context of 3D modeling, particularly within the Shell command, the primary focus is on modifying a solid model to create hollow structures by defining the thickness of the walls. The Shell command typically allows for options like constant thickness, variable thickness, and wall offset, all of which relate to how the material is removed or retained around the interior of the object.

Constant thickness lets you set a uniform wall thickness throughout the entire hollowed object, ensuring that the walls are the same width regardless of changes in the shape of the model. Variable thickness, on the other hand, allows you to define different thicknesses in various areas of the model, providing flexibility for more complex designs. Wall offset enables you to create an offset distance from the surface of the solid to the shell surface, allowing for additional control over the shell's dimensions.

Concentric design, while a relevant concept in modeling, is not directly associated with the Shell command in terms of modifying wall thickness or hollowing out an object. Instead, concentric design pertains more to the alignment and arrangement of multiple features or components in relation to a common axis, which is not a defining characteristic of the Shell operation.