The commanding officer (CO) is the officer in command of a ship or base. Typically, the commanding officer has ultimate authority over the unit, and is usually given wide latitude, within the bounds of military law. In this respect, commanding officers have significant responsibilities, duties (to higher authority, mission effectiveness, duty of care to personnel) and powers (for example, discipline and punishment of personnel within certain limits of military law). A commanding officers may be of any commissioned rank; usually there are more officers than command positions available, and time spent in command is generally a key aspect of promotion, so the role of commanding officer is highly valued, and in theory only goes to the best officers. The commanding officer is often assisted by an Executive Officer (XO) or Second-in-Command (2XO), who handles personnel and day-to-day matters. Larger units may also have staff officers of various responsibilities.