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An agreed set of standards under which an aircraft is allowed to operate dependent upon local weather conditions, etc.

• '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.

From ICAO doc 4444 appendix 2 (Flight Plan):

• I if it is intended that the entire flight will be operated under the IFR
• V if it is intended that the entire flight will be operated under the VFR
• Y if the flight initially will be operated under the IFR, followed by one or more subsequent changes of flight rules or
• Z if the flight initially will be operated under the VFR, followed by one or more subsequent changes of flight rules