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.
Comments
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
