and some controllers use mach nos on high level decents, and some use ias??
All good radar controllers (and some bad ones too) know that a/c fly Mach above certain levels and then switch to IAS once lower. If an a/c descending is given an IAS to fly while still in Mach mode, words to the effect of "when able" are added. It just means that during Mach there is no problem, but one may occur on IAS. Other controllers need to control the mach too, so normally regulate that first then give an IAS to convert to. Basically it all amounts to the particular controller and certain situations.