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The Groups & Files list at the left of the project window is at the root of all the work you do in Xcode. All access to project files and build settings starts in this list. A close look shows that it is more than a hierarchical file list.
The first “group” in the Groups & Files list is the Project group. Every file in your project appears somewhere in this group. I didn’t say that the Project group includes every file that goes into building your product—though it does. The Project group also includes anything, anywhere, that you might want to search, copy, or otherwise refer to. It’s target membership that identifies the files that are ingredients for a product.