ICAO Doc 4444
5.3.4 Vertical separation during climb or descent
5.3.4.1 An aircraft may be cleared to a level previously occupied by another aircraft after the latter has reported vacating it, except when:
a) severe turbulence is known to exist;
b) the higher aircraft is effecting a cruise climb; or
c) the difference in aircraft performance is such that less than the applicable separation minimum may result;
in which case such clearance shall be withheld until the aircraft vacating the level has reported at or passing another level separated by the required minimum.
Your Notes (not visible to others)
This question has appeared on the real examination, you can find the related countries below.