Erm...
The game has a bunch of what if's right.
So for example, when entering the purifier in Fallout 3 we can either just fix it or enter Eden's serum.
The Official Storyline on the other hand can't have a bunch of what if's.
So when Fallout 4 is made we get to know what ending was the official one.
Like for example: Whether the main character according to the Official Storyline blew up megaton or didn't.
And so on and so on.
That is canon, official storyline. (At least that's how I interpret it)

If you want to know more about the fallout lore then yup.
But it's 10 parts long, so expect a very very very long read.

I don't know about the rest of you but I hated the metros. They were hands down my least favorite thing about Fallout 3. Although the D.C. ruins were fun to explore I liked the open wasteland a lot more. I think NV will have a lot more areas suited to my play style.

I felt the same about them.I couldn't stand them being everywhere in the D.C. ruins.

Or Bethesda can simply avoid the subject and keep it ambiguous; which is what they usually do in their sequels. Heck in The Elder Scrolls all of Daggerfall's numerous endings are canon.