Military Occupational Specialties (MOS) are the different classes of Heroic units that players can choose to play as at the start of each game.
Each Heroic unit is capable of earning experience by killing enemies, thereby unlocking its own unique abilities and fulfilling different roles over the course of the game. By default, a player only has access to the Rifleman and the Combat Medic. As a player's Rank progresses in each Rank Set, additional MOS will become available to that Set only (e.g. A player with a rank of Captain on the Commissioned Officer Rank Set but without a rank of Sergeant Major on the Enlisted Rank Set cannot choose Fire Support Specialist as an Enlisted.).
Visit the link for each MOS for an overview of the class as well as guides for performing the roles of that MOS.