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FCTM Adverse Weather:« Configuration FULL, or 3, can be used.
CONF FULL provides better handling capability in turbulent conditions, however, CONF 3 provides more energy and less drag. »
It has to do with gain modulation of flight controls. F3 is very sensitive so F full handles better. There was an accident in Hong kong due to flap jam and go around which explains this. Google it. But climb gradient is an issue for whatever reason then off course F3 is better.