Chief Warrant Officers are the most junior of line officers. They usually serve as Engineering Technicians or Medical Technicians. A Chief WO can also be transporter chief on a starship, for example.
Ensigns often serve as flight controller aboard starships, or even operations officer on small ships. Starfleet Academy cadets can be granted the rank of Ensign temporarily for outstanding grades.
Officers typically spend around two years as a Lieutenant Junior Grade before they are promoted to full Lieutenant.
Lieutenants are the most common comissioned officers. They can perform almost any officer duty on a starship, save for being the commanding or executive officer. In some cases, especially on smaller starships, Lieutenants can even be department heads.
Lieutenant Commanders usually serve as department heads aboard starships or on large starbases. They can also serve as executive officers.
Commanders usually serve as the Executive Officer, Operations Officer or Security Chief aboard a starship. Commanders may command small starbases or minor outposts. Taking the Bridge Officer Examination is the only way for a science or medical officer to be promoted to Commander, pending completion of the other requirements.
Captains are the most senior of line officers. They most often command a Starship or Starbase of medium to large size. Captains may also command planetary outposts or serve as division heads for the Starfleet bases on major worlds.
