Actually this is one of exercises in the book that I am reading, and I have encountered this piece.
The chord symbol given is D7. The chord written is B/D.
How could (D F# B D#) be considered same as (D F# A C)?
I wonder such roughly putting notes together is commonplace in music?
It could be written B/D. It could even be written B7/D. Either of those would accurately describe the pitch content.
However, presuming this is an interpretation from a lead sheet, D7 clearly indicates that the chord is part of a ii-V progression (in G major, the apparent key of this [part of the] piece).
The D7 can be "heard" since the chord contains D-F#-A, and the C was just heard in the previous chord, setting the overall tonality. The B then is the 6 or 13 — a chord extension — which also describes the D# (i.e., Eb = b9).
So the symbol to describe the complete voicing would be D7b9(add13) or, since the C isn't literally part of the chord, D6b9.
Based on the fact that this is from a jazz harmony book from Berklee it seems likely this entire example revolves around using triads and 7th chords to form upper structures of basic chords. Triads used in this way are referred to as upper structure triads. These types of chords are called poly chords. They are usually written like a vertical fraction instead of a slash chord. Your example is based on a B triad over a D7 chord, written like this:
This ends up being a D13b9 chord. The B is the 13, the D#(Eb) is b9 and the F# is the 3. In this case the chord symbol indicates that the D7 chord is played under the triad. In this case you can spell the chord from the root but sometimes poly chords don’t have a clear single chord symbol to define them.
There are a few others here, the 1st inversion Ab triad over D7alt gives you 7,b9,#11(b5), C,Eb,Ab. The 2nd inversion F triad over Bbmaj7 gives you the 9,5,7, C,F,A.
