14 CFR 38.23: Manual information and limitations.

(a) Information in manuals. The following information must be included in any FAA-approved section of a FAA-approved Airplane Flight Manual or combination of approved manual material:

(1) Fuel efficiency level established as required by this part; and

(2) Maximum takeoff mass at which fuel efficiency level was established.

(b) Limitation. If the fuel efficiency of an airplane is established at a weight (mass) that is less than the maximum certificated takeoff weight (mass) used to establish the airworthiness of the airplane under this chapter, the lower weight (mass) becomes an operating limitation of the airplane and that limitation must be included in the limitations section of any FAA-approved manual.