Not sure of the docs and not sure if correct.
According ICAO, aircraft should report leaving and report reaching levels.
Reporting can be reduced. i.e. with Mode C there is not need to report reaching but still must report leaving.
Only reason I can see for this is it makes ATCs job easier. Once an acft has reported leaving a level it maybe allocated to another acft. With no report the Mode C must indicate more than 300 feet from vacated level.
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