On the new-ish 319s, it's FCOM 4.3.25 P107 which says:
REC MAX This field displays the recommended maximum altitude (in magenta), that is computed based on the current gross weight and temperature. It provides the aircraft with a 0.3 g buffet margin, a minimum rate of climb at MAX CL thrust, and level flight at MAX CRZ thrust. This field is limited to FL 398.
If one engine is out, this field displays the recommended maximum engine-out altitude, that is computed based on the long-range cruise speed and assuming that anti-ice is off.
My understanding is that the increase was made from FL391 to FL398 to allow those that still cruise in metres (Aeroflot which bought the 319) to use the 12000M cruise level.
Cheers,
mcdhu