((a)) Whenever the President calls the National Guard of a State into Federal service, he may specify in the call the period of the service. Members and units called shall serve inside or outside the territory of the United States during the term specified, unless sooner relieved by the President. However, no member of the National Guard may be kept in Federal service beyond the term of his commission or enlistment.
((b)) When the National Guard of a State is called into Federal service with the National Guard of another State, the President may apportion the total number called from the Army National Guard or from the Air National Guard, as the case may be, on the basis of the populations of the States affected by the call.