VFR Flights
Except where otherwise indicated in ATC clearances or specified by the appropriate ATS authority, VFR flights in level cruising flight when operated above 3000ft (900m) from the ground or water, or a higher datum as specified by the appropriate ATS authority, shall be conducted at a cruising level appropriate to the track as specified in the Semicircular Cruising Level System (ICAO).
IFR Flights within Controlled Airspace
An IFR flight operating in cruising flight in controlled airspace shall be flown at a cruising level, or, if authorized to employ cruise climb techniques, between two levels or above a level, selected from:
a) the table of semicircular cruising level system; or
b) a modified table of cruising levels, when so prescribed for flight above FL410;
except that the correlation of levels to track prescribed therein shall not apply whenever otherwise indicated in ATC clearances or specified by the appropriate ATS authority in Aeronautical Information Publications.
IFR Flights outside Controlled Airspace
An IFR flight operating in level cruising flight outside of controlled airspace shall be flown at a cruising level appropriate to its track as specified in:
a) the table of semicircular cruising level system, except when otherwise specified by the appropiate ATS authority for flight at or below 3000ft (900m) AMSL; or
b) a modified table of cruising levels, when so prescribed for flight above FL410.
Source: ICAO
See also:
Altitude Regulations
Altimeter Setting
Transition Altitude
Transition Level
System of Flight Levels
Cruising Levels
Semicircular Cruising Level System (ICAO)
Cruising Levels Applicable in RVSM Airspace