Shell Feature References The shell feature does not require selection of "surfaces to remove", as it can hollow out a model without any references.  However, when selecting surfaces to remove, you must press CTRL to select multiple references.

Task 2-8. Create a Shell Feature to hollow out the model.

1. Start the Shell Tool from the feature toolbar.
   1. Drag the thickness handle to 2.
   2. Press CTRL and select the two surfaces shown.
2. Click Complete Feature from the dashboard. 

Selecting Surfaces to Remove