The names of commissioned ships of the United States Navy all start with USS, for United States Ship. Non-commissioned, primarily civilian-crewed vessels of the U.S. Navy under the Military Sealift Command have names that begin with USNS, standing for United States Naval Ship. A letter-based hull classification symbol is used to designate a vessel's type. The names of ships are selected by the Secretary of the Navy. The names are those of states, cities, towns, important persons, important locations, famous battles, fish, and ideals. Usually, different types of ships have names originated from different types of sources. Modern aircraft carriers and submarines use nuclear reactors for power. See United States naval reactors for information on classification schemes and the history of nuclear-powered vessels. Modern cruisers, destroyers and frigates are called surface combatants and act mainly as escorts for aircraft carriers, amphibious assault ships, auxiliaries and civilian craft, but the largest ones have gained a land attack role through the use of cruise missiles and a population defense role through missile defense. Aircraft carriers Aircraft carriers (CVN) have the ability to put most nations within striking distance of U.S. air power which makes them the cornerstone of US forward deployment and deterrence strategy. Multiple carriers are deployed around the world to provide military presence, respond quickly to crises, and participate in joint exercises with allied forces; this has led the Navy to refer to their Nimitz-class carriers as "4.5 acres of sovereign and mobile American territory". "In accordance with customary international law as reflected in United Nations Convention on the Law of the Sea and as discussed in The Commander's Handbook on the Law of Naval Operations and The State Department Cable on the Status of Military Sealift Command Vessels, ships owned or operated by a State and used, for the time being, only on government non-commercial service are entitled to sovereign immunity. Whether in internal, territorial, or international waters, such ships are immune from arrest and search, and exempt from foreign taxation and any foreign regulation requiring flying the flag of that foreign state while in its ports or territorial sea." Former President Bill Clinton summed up the importance of the aircraft carrier by stating that "when word of crisis breaks out in Washington, it's no accident the first question that comes to everyone's lips is: where is the nearest carrier?" The power and operational flexibility of a carrier lie