From the FCOM.
"The soft altitude mode engages when the aircraft reaches the FCU altitude set as the cruise flight level (entered in the F-PLN or on PROG page).
The soft altitude mode corrects minor deviations from the Mach target by allowing a ±50 ft variation from the CRZ FL. This feature improves fuel efficiency and passenger comfort and minimizes the changes in thrust."
So by either going ALT (as opposed to ALT CRZ) or Speed from Mach you are effectively 'stiffening' up the response to BOTH Speed and Alt?
The speed control I can get, but surely a 50ft margin in turbulence might make it more comfortable and less stressful for humans and machine?
Interesting. I knew about soft alt but not about the speed/mach pb.
Cheers