A fleet captain is a senior captain typically in command of a larger starship (a heavy explorer, heavy cruiser, carrier or similar vessel) but who is also tasked with the command authority of more than one vessel. Frequently, a fleet captain is in command of a frontline squadron. In general, this rank is seen as a temporary measure used in case of emergency or fleet engagement, as such a rank is essentially equivalent to that of rear admiral, lower half, but does not carry the privileges of admiralty. A fleet captain could also act as the executive officer to an admiral (generally one of one or two star rank) in command of a particularly large task force.
This rank is sometimes called a fleet commander. In Starfleet Intelligence, an officer of this rank is known as a senior captain.
Rank insignia of Starfleet fleet captains