In tertian music theory, the chords are basically pancaked every other note. These sound nice and harmonious due to the ratio of the frequencies.
In the G Major chord pancake we have G xa B xc D for G-B-D, chord scale numbers 1, 3 and 5. Extending these sonorities continues in the same pattern, next is the 7 chord:
1-3-5-7 | G-B-D-F
Only the highest number is displayed starting on 7, going through 13. These are the extended chords.
Finally, we get to the beautiful 9 chord, G9:
1-3-5-7-9 | G-B-D-F-A
"add2" is signaling to voice the chord as G-A-B-D-F. This moves the A down one octave nestled in between G and B. Essentially, the 1-2-3 cluster is the desired effect.
"sus2" is signaling that the chord begins with an A that will resolve to G or B. 2 can go to either 1 or 3 in this case since they are equally distant apart at a whole step. G to A and A to B.
Resolving to 1 sounds more complete and resolving to 3 is more suspenseful.
