From ICAO Doc-444 PANS-ATM
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.
When the aircraft concerned are entering or established in the same holding pattern, consideration shall be given to aircraft descending at markedly different rates and, if necessary, additional measures such as specifying a maximum descent rate for the higher aircraft and a minimum descent rate for the lower aircraft should be applied to ensure that the required separation is maintained.
Pilots in direct communication with each other may, with their concurrence, be cleared to maintain a specified vertical separation between their aircraft during ascent or descent.