Folks,
Driftdown can be a couple of things.
411A is correct in regard to maximising range for two cases: first, for calculating ETOPS threshold distance (always much further than steady state OEI CRZ suggests) and, second, for fuel efficiency.
The others are correct for the third case of minimum descent angle for terrain clearance.
ITCZ, Oz requires driftdown planning for 3/4 engine aircraft where 2 engines may be inoperative - often more critical than the OEI case for a twin.
Stay Alive