Space Fleets are generally organised into hierarchical groups for specific strategic purposes. Vessels could operate in those groupings in various situations either permanently or over a short period of time.
A unit is one vessel, operating alone. This can range from one Runabout on a survey, to a Federation-class Carrier yet to join its Carrier Group.
Division (Tactical Unit)
A Division is the smallest tactical unit used in Star Fleet, usually comprising of between two and four ships depending on size. For example, a Frigate or Destroyer division would likely contain four vessels, while a Heavy Cruiser division would contain only two. Divisions are almost always grouped together to form Flotillas or Squadrons, though some operate independently. A Division is commanded by the senior Commanding Officer, regardless of rank, and is generally seen as a lead-in to Squadron Command and eventual flag rank.
Squadrons are a small group of generally between two and four Divisions. A Cruiser Squadron would usually be made from two Cruiser Divisions, while a Destroyer Squadron would be composed of three or four Destroyer or Frigate Divisions. Squadrons are permanent administrative organisations, though are usually only deployed together during times of crisis. A Squadron is usually commanded by a Commodore or senior Captain depending on size.
Star Fleet also organises small ships like their Starfighters or Attack Craft in Squadrons; each usually consisting of between six and twelve craft. An Attack Craft Squadron is usually commanded by a senior Lieutenant or Lieutenant Commander
A Wing refers to a collection of small vessels greater than that of Squadron-size, such as an Attack Craft Wing launched from a Star Base or Carrier. In this respect, a Wing is usually formed of between two and five Squadrons, commanded by a Commander or Captain.
Battle Groups consist of an arbitrary number of ships, usually gathered temporarily for a single mission that mostly involved combat, though Battle Groups can exist as more permanent organisations during times of war. Battle Groups of all forms are commanded by a Flag Officer of at least Commodore rank, though in emergencies a senior Captain has been given command.
A Strike Group is a form of Battle Group designed to penetrate enemy territory with a specific objective, such as crippling the enemy flagship, cutting off lines of retreat, or disrupting supply lines. A typical Strike Group would contain a Battleship and a screen consisting of at least one Cruiser Squadron and one Destroyer Squadron.
A Carrier Group is a form of Battle Group consisting of a number of ships formed around a single Federation-class Carrier. A typical Carrier Group would include a Carrier and a screen consisting of at least one Cruiser Squadron and one Destroyer Squadron.
A Task Force was a fleet of ships for a special task such as assault, protection or diplomacy and was generally bigger than the relatively small Battle Group. Like the Battle Group, the number of ships in a Task Force could vary greatly: There were small Task Forces consisting of only nine ships and large ones consisting of several hundred. Task Forces are usually commanded by Flag Officers of Rear Admiral rank or higher, though in some cases they have been commanded by senior Captains.
Star Fleet organises all ships in Numbered Fleets. A Numbered Fleet may consist of several hundred ships, all localised in a specific region of space or operating independently on exploration missions. There are currently at least seven Numbered Fleets operating in Star Fleet. A Numbered Fleet is commanded by a Flag Officer with the rank of Rear Admiral or higher.