FCOM 1
SOFT ALTITUDE
On reaching the planned cruise altitude, ALT CRZ engages and A/THR maintains the speed/mach target.
Two minutes after ALT CRZ engages, if the mach mode is operative, SOFT ALT mode engages. This allows the aircraft to deviate ± 50 feet from the target altitude, thereby minimizing thrust variations and reducing fuel consumption.
This is on all (I think) FBW Airbus types, and is not a "special" mode selected by the crew... but used on every flight where the aircraft is cruising at (but not below) the FMC programmed Cruise Alt...
NoD