An agreed set of standards under which an aircraft is allowed to operate dependent upon local weather conditions, etc.
Legal values
'I': (Instrument flight rules): regulations and procedures for flying aircraft by referring only to the aircraft instrument panel for navigation.
'S': (special visual flight rules): a set of aviation regulations under which a pilot may operate an aircraft
'V': (visual flight rules): a set of regulations which allow a pilot to operate an aircraft in weather conditions generally clear enough to allow the pilot to see where the aircraft is going.
