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I'm not quite sure I understand the question, but will take a stab.
Root Community is a Container of Places and (Placeless content, i.e. private content).
Each place has contents they can store based on their configuration and rules.
Spaces are places that are nest-able and support ACL like permission sets (defined from the top-down).
Group are places that are flat in nature (no nesting allowed) and support a more organic (bottom-up) assigned membership. Add Member, Request to Join, etc...
Projects are a sub-place that can be stored inside a Space or a Group for all intents and purposes.
Contents inherit the permissions from the places they are stored.
That's an oversimplification of Content/Places and relationships...but hope it helps.
There are a series of documents available here that I would recommend for these types of questions:
as the answer may be better illustrated.
Hope that helps.