This "in-universe" document presents a much needed re-interpretation of Starfleet officer breast insignia in STAR TREK: The Original Series, after decades of manuals conveying the incorrect assumption that each Starship was intended to have its own unique emblem.
With that now proven false and the Exeter patch revealed as a costuming error, this system is authored to be consistent with the remaining data — necessarily supplemented with some well-reasoned conjecture.
"Real-world" explanations are contained within the companion article: Breast Insignia - Analysis.
As per the Starfleet Uniform Regulations of 2266, all officers shall wear an authorised insignia in its prescribed orientation on the left breast of their standard service uniforms.
The breast insignia emblem serves to visually identify an officer's current assignment type and category, while uniform colour indicates their overall Division.
Additionally, the insignia of all Starship personnel shall incorporate an embroidered device that corresponds to the wearer's Division.
The evolution of language and tradition are organic processes. Although Starfleet's primary mission is one of exploration, much of its military-style structure and protocol can trace their roots back to those of old Earth navies.
Notably, traditional terms utilising terrestrial words such as "ground", "field", and "marine" endure within the service: their definitions having evolved to encompass such concepts as Starbases, Starships, and the environment of outer space.
Authorised Insignia - Emblems
Starship - Commissioned Officer
Starship personnel wear the prestigious "arrow-head" patch, which forms the shape of a Greek letter delta and is perhaps the symbol most widely associated with Starfleet throughout the member worlds of the Federation.
Starships are at the heart of Starfleet's primary mission: "To explore strange new worlds ... to seek out new life and new civilizations ... to boldly go where no man has gone before!"
When interstellar conflict arises, Starships are sent to the front lines of defence. On deep space assignment, these formidable vessels and their crews often find themselves acting as the sole representatives of the Federation's laws and customs, well beyond the range of the nearest Starbase.
Only the best are accepted into Starfleet Academy. After rigorous mental and physical training, only the best of the best graduate with a posting to Starship duty — with many hopefuls aspiring to one of the dozen Constitution-class heavy cruisers that signalled a new era in design and capability.
Devices are displayed upon these insignia, as detailed below.
Ground Assignment - Commissioned Officer
All officers on ground assignment wear a 10-spoked gold and silver metallic "starburst" insignia: be they Ensigns serving on Federation outposts, Lieutenants in the Judge Advocate General's office, or Commodores commanding Starbases.
In the public mind, this emblem is most often associated with the admiralty of Starfleet Command, being its most high-profile exponents. In truth, it is worn by the widest breadth of ranks of any Starfleet insignia.
While ground assignment may not carry the inherent adventure or prestige of Starship service, the numerous ships of the fleet would quickly deteriorate if not for an extensive command and support structure. It is the duty of these officers to provide that direction and support.
Field Command - Flag Officer
Flag Officers on field assignment wear an ornate gold and silver version of an "arrow-head" insignia patch, with a grandiose curve that spirals upward rather than forming a delta. (N.B.- Flag ranks are all those Commodore and above.)
Given Starfleet's edict to act only in a defensive military capacity, flag officers holding field commands are rare. The overwhelming majority of commodores and admirals serve in administrative positions on ground assignment. Nonetheless, these ranks retain a defined ability to command squadrons and other multi-ship forces under a traditional naval command structure.
Moreover, Starfleet Command may pre-emptively prime key Starships for use as flagships in the event of escalating hostilities, by assigning a Commodore rather than a Captain as Commanding Officer. In this configuration, the rank of Commodore returns to its original meaning of "a senior Captain whose authority extends to multiple ships" — although that authority is largely honorary until circumstances necessitate otherwise.
Devices are not displayed upon these insignia — officers are at the squadron/force level of command, and thus not considered Starship personnel in the strict sense of the term.
Starfleet Academy - Cadet
Cadets are considered on assignment to Starfleet Academy (San Francisco, Earth, Sol), and wear a "starburst" insignia that is a recognisably smaller version of those worn by commissioned officers on ground assignment.
These Midshipmen have not yet achieved commissions, but as officers in training are taught to give and receive orders as such. Thus, insignia is prescribed.
Devices do not apply, nor do Division colours. Cadets are not yet considered to be on assignment to any specific department, and are all equally identified by their distinctive silver-grey service uniforms.
Merchant Marine - Licensed Officer
With the "arrow-head" emblem reserved exclusively for Starfleet's own Starship personnel, licensed officers operating civilian-owned space ships in the merchant service bear their own unique insignia.
The Merchant Marine is responsible for transporting cargo and passengers amongst the worlds of the Federation, and in times of crisis can be called upon to deliver personnel and materiel as an auxiliary to Starfleet.
Devices are displayed upon these insignia, albeit with variations in design to those contained herein. Starfleet's Division colours do not apply. The Merchant Marine utilises service uniforms reminiscent of Starfleet's pre-2266 style, according to their own separate uniform regulations.
Divisions, Line Officers, and Staff Officers
Starfleet officers are categorised into three general Divisions, which further sub-divide into specific departments, according to the needs of a given unit.
Those in the Command Division are trained as Line Officers to exercise general command authority.
Those in the Sciences Division or Services Division are primarily specialists in their career fields, trained as Staff Officers to exercise command authority only within their own specific departments.
N.B.- This is not meant to suggest that the Command Division doesn't contain its own specialised departments, or that Line Officers aren't specialists. Conversely, Starfleet provides that Staff Officers who aspire to place within the chain of command for their overall vessel may elect to undergo the additional necessary training, thus qualifying as line officers even while remaining in their current department.
Division Colours and Devices
Divisions are ascribed their own specific colours and devices.
The Division of any Starfleet officer may be identified by the colour of their uniform; the Divisions of Starship personnel may additionally be identified by the device embroidered upon their insignia, as follows:
Command - Gold / Star
The Command Star is paired with a gold service uniform* and identifies officers with general command authority, responsible for the administration, operation, and safe passage of their vessel.
Most prominently, this includes the ship's Captain and First Officer, as well as the Helm, Navigation, and Weapons departments.
* N.B.- Hearkening back to traditional military colours, Command dress uniforms are green. The optional wraparound tunic variant – a special privilege afforded only to commanding officers of Starships – follows suit. Command is the only Division ascribed two official colours: gold as primary/default and green as secondary/special.
Sciences - Blue / Globe
The Sciences Globe is paired with a blue uniform and identifies specialist officers working in any branch of the sciences: including the ship's Science Officer, as well as the Life Sciences and Medical Departments.
Services - Red / Spiral
The Services Spiral is paired with a red uniform and identifies specialist officers responsible for the maintenance of the vessel and its equipment, or for the provision of any service that is auxiliary to the direct command and operation of the ship.
Some of the most notable inclusions are the Engineering, Communications, and Security Departments, as well as the Captain's Yeoman.
Nurse - Light Blue / Red Cross
Nurses are specifically identified by light blue nursing uniforms and a Red Cross device on their breast insignia. Initially, this was embroidered over the Sciences Globe, but later became a stand-alone device in its own right.
While ostensibly listed as a fourth Division, Nurses serve under the auspices of the Medical Department. Officially, Nurses are considered auxiliaries to the Sciences Division: forming their own special corps within Starfleet.
Ensign-grade nurses have been accelerated into the service, being granted commissions on the basis of their necessary medical skills, and the results of physical and psychological fitness testing. As such, Nurses are limited duty officers that are eligible for promotion only within their own corps.
Prior to the uniform revisions of 2266 – wherein red replaced beige as the Services Division colour for improved visual distinction – it was not uncommon to notice Starship officers wearing insignia devices that did not correspond to the Division colours of their uniforms.
Once permitted under regulations with the idea of enabling multi-disciplined officers to denote training in areas outside their current assignment, this practice was considered either confusing or unnecessary by many, and is being phased out in favour of consistent device/colour pairings.
For example: a ship's First Officer who also serves as its Science Officer must now fully elect for either a Command or Sciences uniform, according to what it is deemed best represents his function.
For a "real-world" explanation of the information contained herein, please see the companion article: Breast Insignia - Analysis.