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")
"Spacecraft Duty Insignia"