Within the STARFLEET Marine Corps, your unit has the opportunity of choosing a Branch of Service or BOS.  Choosing a branch limits some uniform options and insignia choices for members of the unit, but does not affect activities of the unit as a whole.  The following are available Branches of Service are available for units:

• Aerospace:  Aerospace personnel fly and maintain aerospace fighters, attack craft, cargo transports, and other Marine trans-atmospheric craft like drop shuttles and medivac transports. Other members of this branch coordinate and assist pilots in their duties. 
• Armor:  This branch consists of Artillery, Mechanized Infantry, Surface Engagement, and Cavalry. Artillery units provide fire support and air defense to ground units. Mech Infantry provides fast and protected transportation for infantrymen. Surface Engagement uses heavy firepower in the direct assault or defense. The Cavalry provides reconnaissance and security for other ground forces. This branch is often referred to as a sister service to Mech, because of the similarities in their operational doctrine. 
• Combat Engineers: The Support Branch is the SFMC's largest and most complex. Combat support and combat services support functions ranging from military intelligence and signal corps, logistics and research & development and are encompassed by the Support Branch.
• Infantry:  Infantry is the fundamental combat force of any military and the cornerstone of the Corps. There are two basic types of infantry in the SFMC: Light Infantry and Powered Infantry. Light Infantry resemble their historical counter-parts of the late 20th Century-well equipped, smart, and agile. Powered Infantry adds a new dimension to the battlefield. Wearing self￾contained, heavily armed, and armored powered suits, each carries firepower nearly equal to an entire squad of Light Infantry
• Maritime Operations:  - If you follow the traditions of the Marines back to its very beginnings, there is one thing that differentiates it from other services in the fact that they rode ships to where they needed to go. After many long years, the SFMC has returned this proud element to the Corps. The Maritime Operations branch is responsible for all seaborne operations.
• Medical:  Members of this branch provide first aid and surgical support of all types during combat operations.
• Mech:  Mech are large, heavily armed, and armored humanoid vehicles capable of a formidable variety of combat applications from assault to air defense. Not as fast as armored units on open ground, they still carry a tremendous amount of firepower, and can do many things that traditional armor cannot. They have a limited but vital application on the modern battlefield, and Mech forces are usually elite units with high esprit de corps.
• Special Operations:  Because the SFMC has so much space to cover and only so many rapidly deployable resources in any given sector, the Corps relies more heavily on its Special Forces than perhaps any other military force in history. These "force multipliers" can conduct  unconventional warfare that can augment, and sometimes even substitute for, conventional ground force deployment. Members of this branch perform unconventional and often classified duties like small unit raids or hostage rescues. These duties often involve working without friendly support and inside hostile territory.
• Support:  The Support Branch is the SFMC's largest and most complex. Combat support and combat services support functions ranging from military intelligence and signal corps, logistics and research & development and are encompassed by the Support Branch.

An important thing to remember - A member of a unit does not have to have the same MOS as the BOS of their unit.