The Merchant Marines or Merchant Service was a group within the Federation that operated various survey vessels and freighters.
Carstairs, of the USS Enterprise, formerly served on a Merchant Marine freighter in the Alpha Carinae region. His ship had at one time visited Alpha Carinae II to conduct a geology survey of the planet for a mining company. (TOS: "The Ultimate Computer")
The SS Beagle, commanded by Captain R.M. Merik, was a vessel operated by the Merchant Service. Merik joined the Service after dropping out of Starfleet Academy during his fifth year. The training requirements to command a spaceship in the Merchant Service was less than that of a starship captain. (TOS: "Bread and Circuses")
William B. Harrison, who was trained as a flight control officer, was also a member of the Merchant Service. (TOS: "Bread and Circuses")
![Antares patch](https://cdn.statically.io/img/static.wikia.nocookie.net/memoryalpha/images/6/6e/Antares_patch.png/revision/latest/scale-to-width-down/180?cb=20211223224414&path-prefix=en)
"Spacecraft Duty Insignia"