A quote that will only add more confusion to the concept;
Eddington: Let's face it..People don't enter Starfleet to become commanders... or admirals for that matter.
It's the Captain's Chair that everyone has their eye on. That's what I wanted when I joined up. But you don't get to be a captain wearing a gold uniform.
Sisko: You could always transfer from Security to Command.
In "Disaster" there was a red shirted Lt Jr Grade who was in command, even though Troi outranked her.
Later after she is killed, and they're wondering who gets command, Obrien states that it is Troi, because she holds the rank of Lt Commander.
And yet Ro who was a red shirt Ensign was automatically passed over. More confusion.
P.S, I think they might chose Dax to captain the Defiant, particularly because
of her science experience--for that particular mission.