I never got the MAINTAIN bit in the US phraseology.

What is anyone going to do other than stay at the level once they reach it ?

As for "when ready" clearances, that normally shows the controller is well ahead of the game.

I'm curious to know how it works in the US or European about descent or climb instruction by ATC in a STAR or SID?
If the ATC says: ABC descend to 3000 feet but you are in the middle of the STAR at 5000 feet and there is an altitude constraint at the next waypoint'' at or above 5000 feet ''15 nm ahead of you?
Do you clarify if there is any restrictions before initiating the descent?
In China, when ATC asks you to climb or descent they expect you to do it without delay and with an high ROC or ROD. You don't need to clarify if there is any restrictions. I can't find it written anywhere but in the 6 years + flying in China never the ATC asked us to comply with the altitude restrictions depicted on the SID/STAR.

Have a look here:

DOC4444

Under:
6.3.2.4 CLEARANCES ON A SID
And:
6.5.2.4 CLEARANCES ON A STAR

Thank you Jmmoric! I'm assuming ATC will follow strictly the wording specified in Doc 4444. In China they only use '' Climb'' or '' Descent'' in all cases.

I have no clue why?

Cannot get access to AIP China.