[AlexF]

The word descend at pilot's discretion has always confused me. To me, I it means "stay where I want, until I want lower" however to most ATC that I come in contact with, they want me to start my decent fairly soon despite their kind words of "pilots discretion." I believe technically it is like a cruise clnc where the pilot may start his decent when he wishes, decend at any speed, temporarly level off at an altitude if desired and then decend some more. However, once an altitude has been vacated, the pilot may not return to that altitude.

Technically, the pilot in command my do what he deems necessary for the safety of the flight. However, if ATC tells you to decend, it may be for several reasons like prep you for the approach, service volumns of VORs, traffic etc. If ATC wants to help you get ready for an approach, they'll probably have no problem with you staying where you are, but if they want/ need you to descend (then they probably won't use at pilots discretion either) and you don't feel comfortable, then I'd tell ATC why you don't want to decend and offer solutions like climbs, turns, have conflicting traffic move etc.