Minimum flight weight.
The 707 had one, and this had to do with the minimum amount of fuel that was required in tanks 1 through 4, to ensure no tank outlet unporting, during takeoff/initial climb.
Bangladesh Biman found out the hard way about this. On departure SIN enroute KUL, two engines flamed out just after rotation, and the airplane crashed just off to the side of the runway at Paya Lebar.
Everyone walked away but the airplane was a total loss.
I know because I was scheduled to operate that night to BAH.
We were delayed for five hours whilst the airplane was dragged further from the runway.
Short story...if it's in the AFM, you had better observe, or else....bad things can happen.