Been working on America by Paul Simon. The bridge calls for a Bbmaj7 which is new to me. The diagram calls for only the 4 middle strings to sound...A1, D3, G2 and B3.

I am not great on theory, and wondering why not Barre across the whole first fret, which would add 2 F's....there is already an F in there so it must fit as a "5" right? Also is the D note the "3"? Just wondering if someone can break down this chord for me as I'm trying to understand chord theory.