The following is assuming US FAA and IFR.
This question stems from a question on CRUISE clearance. Section 4-4-3.d.3 of the 2021 AIM says that in a CRUISE clearance
Climbs/Descents within the block is to be made at the discretion of the pilot.
I've never been given a CRUISE clearance, but I have been given a Block altitude assignment before (e.g. 8000-12000). I could climb and descend to my heart's content without telling ATC every time I changed altitude within my assigned block. In fact, I've been voluntarily given a block altitude assignment by ATC just to shut me up after asking for too many altitude deviations for weather.
The AIM says about CRUISE clearance,
However, once the pilot starts descent and verbally reports leaving an altitude in the block, the pilot may not return to that altitude without additional ATC clearance.
Some of my fellow pilots interpret this to mean that you must report to ATC when you descend, but the way I read this is that you don't have to report leaving any altitude in a Block until you are finished with that altitude or the block, and once you descend AND report leaving an altitude, then that signifies to ATC that they can assign that vacated altitude or block to other aircraft. However, none of this appears to says to me, that you have to report descending within the block if you intend to climb back up in the block later.
In other words, don't report to ATC that you're descending until you are done with the block. For example, if I'm given 8000-12000. And I start my final descent and I know I'm not going to be climbing back up then I'll tell them Cessna 123 10000 descending 8000. Then ATC can assign the 10000-12000 block.
When given a CRUISE clearance (which includes a block altitude assignment from MEA to CRUISE altitude) where does it say that a pilot must report altitude changes (climbs or descents) within the block altitude. If not, then where does it say you don't have to report altitude changes.
Is there an FAA publication that provides more information on Block altitude assignments and Cruise clearances besides the AIM?