It's pretty restrictive actually. NW has no popular equivalent of SWUs or VIPOWs and if you want to upgrade with miles, you have to purchase certain fare classes. I suppose the fare class restrictions aren't that bad to most places, but to/from China and HK, only Y and B qualify. The irony is that most Au and Pt elites I know in China get operational upgrades more than half the time because Y always overbooks and there aren't a whole lot of NW elites over here. Contrast with United: Y also oversells frequently, but lax upgrade rules generally result in throngs of pax who are willing and able to pay to ride up front. Therefore, operational upgrades are rare on UA.