In my feeble mind it simply implies whether the chord is played in a 'stretched' inversion (a wider range of notes on the keyboard) or tightly (think of Dm9, does the 9 happen in the middle of the chord, actually the second, or up higher where it is truly a 9th).
I think of it as 'is my hand closed on keyboards when playing this chord, or stretched out and open'?
One means the 9th is played right beside the 3rd, the other means it is an octave higher.

I may be wrong, and I have been plenty of times, but that is how I interpret it.
Hopefully others chime in, maybe I can learn something.


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