d246
This is an oldie, basically you comply with an ATC clearance....any subsequent clearance by the same or different controller will have the effect of cancelling the previous clearance or restriction.
Number of things to add......in any new clearance or instruction the controller may state that the previous restriction still applies, or may re-state or amend the restriction. It may be that the previous restriction is redundant through circumstances, or its earlier imposition has resolved the earlier reason for it.
Some restrictions are essential to enable massive traffic through-put. Your example was not dissimilar from the need to descend traffic to FL200 10 before MONTY on the MIRSI 1A STAR.....this is because the sector you will be working expects you to be at that level at or before that point...but the sector above that knows nothing about you..nor does it need to..it is full with its own traffic.
Here's an example going the other way:-
"climb FL270 level 20 miles before CUMRI"........why?
--because the sector 5 controller needs you to be FL270 or above so that he/she does not need to co-ordinate with sector23 [Bristol sector]----why?----because sector 23 will climb London TMA out-bounds to FL260 and transfer them to sector 8 and not to sector 5-----why?---because the use of these simple sausage-machine procedures enables us to handle 147% more traffic than if we had to co-ordinate every movement individually.
IF you are climbing well and it is obvious that you will meet or exceed the restriction then there will be no need to repeat it in subsequent instructions.
A few examples of procedure restrictions which enable huge volumes of traffic to be handled safely:
Levels used in the CPT/Woodley area
110 -Solent clutch airfields EGHI/HH etc from North
120-Solent clutch from West
130- London TMA outbounds west
140-London TMA inbounds from west
150- EGKK etc inbounds from North
160-inbounds to certain military airfields
170-northbound traffic
180-TMA inbound descent level from west to EGSS/GW etc
180-TMA eastbound climb-out from Bristol sector airfields
190-210- from midlands area southbound
210-TMA climb-out westbound from EGSS
220-westbound descent level for Bristol sector airfields
230-Cardiff area climb-out level eastbound
240-not used due proximity of military crossing corridor
this is just a simplified approximation, but it does explain why there are so many peculiar restrictions; many of which are shown on STAR charts as "descent planning". For restrictions at MONTY look up MIRSI 1A or even KEGUN 1D. For ease of traffic handling some airfields without STARS are handled in the same way....example..EGNH[Blackpool]/ EGNM[Leeds] will be handled the same way if coming from the south.
Traffic at non-standard levels will be co-ordinated individually between sectors.