Study for CFII
That must be it, under 91.103(b)(2) "... aircraft gross weight..."
We really drill that acronym into students and yet I have been unable to find a specific reference -- unlike the other parts of AR(R)OW -- to actually have the weight and balance documents on board. For example, 91.103(a) and (b) require checking several things but II have never been taught, nor can I find int he regulations, the requirement to keep a physical copy of the runway length or wx reports on board.