Refer to figure.
ICAO Doc 8168 Volume III - Aircraft Operations
Chapter 1 - DEFINITIONS

A pressure type altimeter calibrated in accordance with the Standard Atmosphere:

a) when set to a QNH altimeter setting, will indicate altitude;

b) when set to a QFE altimeter setting, will indicate height above the QFE reference datum; and

c) when set to a pressure of 1013.2 hPa, may be used to indicate flight levels.

Altitude. The vertical distance of a level, a point or an object considered as a point, measured from mean sea level (MSL).

Flight level (FL). A surface of constant atmospheric pressure which is related to a specific pressure datum, 1013.2 hectopascals (hPa), and is separated from other such surfaces by specific pressure intervals.

• QNH (different every day) → Altitudes in relation to mean sea level.
• QNE (1013.25 hPa) → Flight levels in relation to the 1013.25 hPa datum.
• QFE (different every day) → Vertical distance using the pressure at the airfield as a reference.

Note: At the cruise phase of flight, all the aircraft need the same pressure reference (1013.25 hPa) to ensure safe separation betweeb them.