Starfleet Line Officers
Starfleet Line Officers represent the bulk of the members of Starfleet holding officer ranks, with responsibilities ranging from the command of starship to junior staff positions on minor outposts. Line officers are subordinate to flag officers and have authority over more junior line officers and all non-commissioned officers. There are eight line officer ranks.
Fleet Captain
Fleet Captain is the highest line officer rank in Starfleet, held by senior starship captains commanding task groups or task forces as well as senior administrators or aides within the numbered fleets and at Starfleet Command. The paths leading to fleet captain vary considerably from officer to officer, as it is often a late career reward for experienced staffers, but a transitional step for starship captains on the fast track to flag ranks. Indeed, some officers are promoted directly from captain to commodore, which makes fleet captain rarer than both the ranks above and below it. The authority carried by fleet captains is highly contextual; a senior captain in command of a small task group would only have authority over their direct subordinates, while a Deputy Director of Internal Affairs might have sweeping fleet-level powers.
Captain
Captain, is as the name suggests, the rank assigned to those officers fortunate enough to be granted the command of a starship. While lower ranking line officers do also hold commands of some smaller starships, the rank of captain carries with it additional prestige and authority, as well as greater access to sensitive information. Starfleet captains are also empowered as ambassadors for the United Federation of Planets, with the ability to negotiate on behalf of the Federation Council.