This is how I understand it.
QNH is the QFE (presure measured at the airport) adjusted to sea level using the ISA model.
QFF is the QFE adjusted to sea level using the actual temperature. I have heard that it is used by the met people and also helicopter pilots.
I have a question then.
The DA(H) on an approach is corrected for very low temps.
If you are landing at a very high airport the QFE is still quite accurate whereas the QNH might be very wrong.
Do you still add the same correction to DA and DH???
FIS.