These missions are triggered by joining the elite organization(s) of a various faction.
While the basic campaigns are meant to emulate the 'General War' period of the Star Fleet Battles timeline, the main storyline of Starfleet Command spans several unique faction-based missions, focusing on the disappearance of the Organians. Starfleet Command is an open world game a player's actions during a mission will affect the game world (i.e, if the player succeeds, their chosen faction also succeeds).