as follows: Chairman of the Joint Chiefs of Staff Vice Chairman of the Joint Chiefs of Staff When compared to each other, seniority among the service heads is determined by date of when the officer assumed office. Externally, the standing of each service head is determined by the date of the creation of the position as follows: Chief of Staff of the United States Army Chief of Naval Operations Commandant of the Marine Corps Chief of Staff of the United States Air Force Chief of Space Operations Commandant of the Coast Guard Chief of the National Guard Bureau The officers in charge of the Unified Combatant Commands are considered "operational officers" while the standard military hierarchy is administrative. For instance, the Chief of Naval Operations, who would most likely be senior to a naval admiral in command of the United States Pacific Command, would not be able to issue direct orders to said commander since operational chain of command is separate from regular administrative military hierarchy. Military seniority, within itself, would not be affected. Tactical and operational seniority Tactical seniority, also known as "battlefield seniority", is the manner in which a senior officer in command of a given tactical situation is determined. For instance, within the United States Navy, groups of ships performing exercises together will have one ship designated as the tactical senior unit. The commander of said ship is the senior tactical officer and may in fact be junior in rank to the other officers of the tactical group. For multi-national exercises, such as the Sharem event in South Korea, ships of foreign nations are sometimes given tactical seniority and thus may issue routine movement orders to United States vessels. Actual combat would fall under the Task Force system, in which a United States admiral, with clear seniority, would take command over all vessels. Groups of Army units, especially in active combat, may be placed under tactical command of any officer, regardless of rank seniority, for completion of a single mission. During World War II, the term "mixed unit" was commonly used to denote military formations created from several other smaller units, most often "on the spot", due to operational confusion and the need for a single battlefield commander to take authority over all units physically present. Army Air Force bomber groups operated on a similar principle, in that tactical command could pass to officers who were not