Roddenberry's quote was that, in Starfleet, "... rank is more like a job description".
Ah, thank you.
So then it does fit that Kirk would have a different title to command a starship than he would to serve as Chief of Starfleet Operations.
Doesn't really explain Commodore Decker commanding the Constellation in TOS though. Why shouldn't he be a Captain too, unless he was a fleet commander of some kind?
Indeed, it makes me wonder if Kirk could have retained his Admiral rank and still commanded the Enterprise if he were commanding, say, a task force?
Since the rank of Commodore is usually the "captain" in-charge of a fleet of ships (with each ship still having a captain), I'd assumed that commodores would usually have a ship of their own.
Not trying to nitpick, just spitballing here. It seemed rather clear in 'Doomsday Machine' that Decker commanded the Constellation. He even recorded the Captain's Log, which seems to preclude having a subordinate captain aboard that died with the rest of the crew. I also wonder how the rank of 'Fleet Captain' would come into play? I imagine a Fleet Captain might serve as Captain on the flagship of a Commodore. We also saw Commodore Wesley apparently commanding the Lexington in 'The Ultimate Computer' but it's possible he was only directly commanding for the wargames. It's also possible there was a Captain just off screen someplace.
If it was a task force like the one that met the Borg @ Wolf 359 or in the Terran system, then an admiral is usually the one in charge (ex. Admiral Hanson, and Admiral Hayes). At least in the TNG/DS9/VOY time period...
How, also, would you differentiate 'fleet' and 'task force'? I would think task force would be smaller and fleet (as in, Third Fleet) larger. Of course, it also depends whether there are still Commodores by TMP or TNG, or if they were renamed Rear Admiral. I don't recall for sure whether the TMP/TWoK rank scheme included Commodores?