Hi Frederick,
re:
> So two faces can share an single edge OR two faces have edges that, not sure how to
> put this, exist in the exactly the same space?
Yup, that's correct. There can be one common shared edge between two faces. This is called a "joined edge" and to have a solid you want to have all edges be in this state.
If two faces are touching at an edge but it isn't joined, then there are 2 separate "naked" edges there that are coincident.
> I'm guessing a face must be, for lack of the correct term, "enclosed" by edges?
Yes, a face has "trim boundaries" (or "loops") on it. Each loop consists of one or more edges forming a closed outline.
A face can have one "outer loop" and any number of "inner loops" making holes.
- Michael
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