The Minor 9 Arpeggio is another one of my favorites and although I don't use it for as many Substitutions as the Minor 6 arp, it still has a "One of a Kind" sound that is key in all aspects of my playing.
You can play this arpeggio over Aeolian AND Dorian which makes it a very useful Minor Arpeggio. It has a bit more serious and dark quality compared to the Minor 6 and sounds great over all types of Jazz, Blues, and Rock.
It's also a great Substitution over it's Relative Major Chord as it contains the notes of a Maj 7(add6) without the 5th. So if you were in the key of C Ionian, you could play an "A Minor 9" over a "C Major Chord" and a "D Minor 9" over an "F Major Chord". So that, in addition to using it over Dorian and Aeolian chords, gives you some serious mileage out of one arpeggio.
Finally, I use this the same way I do the Minor 6 arp a half step above a V chord resolving back to a I chord in either a ii V I or ii V i because it contains the Color Tones of a V7b9#9#5 chord which is a seriously Altered sound. It only lacks the b5 to be considered "Fully Altered" but I prefer to partially Alter my V chords as less Alterations give it a stronger identity in my humble opinion. This also gives you the option to switch them up the next time around. It's good to learn how each Altered Note sounds individually when played in a Dominant Chord so that you can achieve Intent in your playing as opposed to Chance. Many players still sound great when only achieving the latter as a good knowledge of Music Theory can mask a lack of the former, but when a great player is playing with Intent, it's a thing of beauty!
Head to the "Lesson Comments" section below and let's talk about this stuff! These aren't simple concepts and they cannot be fully digested in a 5 minute lesson. It is key to discuss and to listen to how other players apply this to all different aspects of music. If you do not, you will merely be able to recite the material at best. We wanna make players out of you and the only way to do so is to hang out with other players!