Hey Slam_dunk, hope that following from ICAO DOC 4444 will be usefull:
After an aircraft has received the clearance for an approach AND has become established on the final approach course (which means, that radar vectoring terminates at this point), the pilot may descend on own discretion to the altitude stated on the IAC for that type of approach.
So if – for instance – the IAC for an ILS approach prescribes to intercept the glide slope at an altitude of 5000 ft but the radar controller issued the approach clearance (e.g. for separation reasons) in 6000 ft already, the pilot may start its descent to 5000 ft - whenever he likes - after he has become established on the localizer.
That however may cause trouble for the radar controller, since the separation could be endangered.
Hence it is good practice to advise the pilot to start descend not before reaching the glide slope:
“ABC123, TURN RIGHT HEADING 060, DESCEND ALTITUDE 6000 FT”
...Readback from the flight crew...
“ABC123, cleared for ILS approach runway 09, LEAVE 6000 ft on the glide, report established”
References:
ICAO Doc 4444 chapter 8.9.4
ICAO Doc 4444 chapter 6.7.3.2.3
ICAO Doc 4444 chapter 6.5.3.5