An enharmonic chord, and it can resolve many ways. IN 4 iterations. I usually view it as passing to an Am or Dm. I think it's the way your think of all dim7 chords. Then there is the 9th theory, and that's some complicated and makes me shake my head.

I'll have to get out my wife's music books after and have a peek. I'd ask her but when she forgets those 15 years of theory culminating in 4 an university, I get a dirty look when I ask a hard one.

There is also a world of difference in it's treatment from classical to jazz in my mind.

Jazz to me spends more times with resolving in unpredictable ways, which can lead to interest. And the chord can mess with one's head using triads and the inversions.

Somewhere I hear the C#dim7 and F#dim7 going back and forth in classical music but I just don't remember where. I'll have to mess with that and get back to you if the light comes on.

Jazz was part of my new year's resolution, for the 5th time, it seems to go in circles.
Gee that was obtuse.

John Conley
Musica est vita