Actually some acoustics too. Joni Mitchell used a ton of alternate tunings

Some electric guitar players use different tunings.

It’s used more often in jazz and Real / Fake books. It simply means “minor”. D-9 = Dm9 Chord formula 1 b3 5 b7 9 https://www.guitar-chords.org.uk/d-minor9-chord.html Sometimes you will see it...