ICAO Phraseology Reference Guide- ALL CLEAR AGC safety initiative.
Flight Levels
Flight levels below FL100 are referred to as two digit numbers e.g. Climb flight
level eight zero to reduce the risk of confusion with a heading instruction eg.
heading zero eight zero.
Flight levels 100, 200 and 300 are often confused for 110, 210 and 310: special
care should be taken when enunciating ‘zero zero’.