Recommended maximum altitude (REC MAX)
The recommended maximum altitude is the lowest of the maximum altitude that :
The aircraft can reach with a 0.3 g buffet margin
The aircraft can fly in level flight at MAX CRZ rating
The aircraft can maintain a V/S of 300 feet/minute at MAX CLB thrust
The aircraft can fly, at a speed higher than green dot, and lower than VMO/MMO
for which the aircraft is certified.
The REC MAX altitude is displayed on the PROG page.
Anti-ice is not taken into account for this computation. Refer to QRH graphs if icing conditions are expected.
A maximum altitude using a 0.2 g buffet margin is also computed. It is not displayed, but the system uses it to limit CRZ ALT entry.
Hope it helps!