ICAO Doc 8168 Volume 1
2.1.4 References to vertical position
2.1.4.1 The vertical position of aircraft operating at or below the transition altitude shall be expressed in terms of altitude. Vertical position at or above the transition level shall be expressed in terms of flight levels. This terminology applies during:
a) climb;
b) en-route flight; and
c) approach and landing (except as provided for in 2.4.3, “References to vertical positioning after approach clearance”).
Note.— This does not preclude a pilot using a QFE setting for terrain clearance purposes during the final approach to the runway.
2.1.4.2 Passing through the transition layer
While passing through the transition layer, vertical position shall be expressed in terms of:
a) flight levels when climbing; and
b) altitude when descending.
2.4.3 References to vertical positioning after approach clearance
After approach clearance has been issued and the descent to land is begun, the vertical positioning of an aircraft above the transition level may be by reference to altitudes (QNH) provided that level flight above the transition altitude is not indicated or anticipated.
Note.— This applies primarily to turbine engine aircraft for which an uninterrupted descent from a high altitude is desirable and to aerodromes equipped to control such aircraft by reference to altitudes throughout the descent.