As an amateur guitar player, I didn't have much use for 9 chords. However, they sound wonderful and become surprisingly manageable on a ukulele with its four strings.
If you check the chord building chart from the "Chords, and how to make them" note, you'll see that a 9 chord is just a 7-th chord with the 9-th note of the scale added. Think '2', but an octave higher.
Consider three cases:
This is a moveable chord. For example, when you move your index finger one fret up the neck to (2,3,2,4) and you've got B9!
This is a moveable chord. For example, when you move your index finger down the neck and let the nut take its place, you have C9=(0,2,1,0).
This, too, is a moveable chord.