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if it get confusing, you can always open the scematics view and that will show you how all your parts are linked in a nice littel tree graph thing. It's also nice while your building your hiearchy. grouping can also lead to problems, because if you have something linked to that object with say reactor or slighttracer shadows or what not, and you want to edit your model you have to ungroup it and it will break all your reactor and lighting links to the group.

Cheers!