Difference of meaning, a term 'by', 'cross'
 

Thanks for visiting
I want to know the difference of meaning, 'by', 'cross'.
For example, "Descend FL100 by Waypoint", "Descend FL100 cross Waypoint at FL100"
And want to know the difference of basis.

Desend by would be used when you are inside controlled airspace, and they want you to be below a certain level by a waypoint, usually because of traffic at a higher level coming the other way.

Cross would be used when crossing an airway or other airspace, and they would usually want to give you a set level for that.

Cap 413 chapter 3 for level by which is just general level reporting
Cap 413 chapter 7 for cross at which is flights crossing airways

CAP413 is a UK Doc and not very relevant in South Korea! The ICAO Doc is 9432.
means that you have until the waypoint to be level, whereasmeans that you are cleared to descend to F100, but should be level before the waypoint so that you cross it level. How far before the waypoint is not clear!