The geometry shell is a copy of an item in your scene. You select the item you want to copy and then go to Create - New Geometry shell and select an option, ie *copy selected item* and hit ok. The copy is not posable but has all material zones of the original. It loads semi transparent so you have to set opacity back to 100.
ETA: how embarrassing ... the instancing thingie is right above the geometry shell thingie in the Create menue and I didn’t see it. LOL