Jeez, I hate to be flamed. 52 year old man with already burning hemmorroids.
If you issue a speed, I think it must be complied with.
The problem as I see it is with what altitude one transitions to IAS versus mach.
My aircraft transition via the FMC at FL 260. It is a Boeing product. God knows whatever the Frogs are doing.
If you assign a speed for the descent, then give a transisition FL to switch to IAS from Mach.
I re-read the FOM pages every time I fly, but it seems to change all the time, depending where I am going.
No book in front of me at this hotel, but in the US I think the FL is 310 to assign speed in IAS versus Mach....(Going down!)
I think that the transition altitude (Not altimetry) for Mach versus IAS is 260, in Europe, different in other areas of the world...
I'm no longer in the building flight time mode. The Barber pole suits me find for descents..........