usable fuel must be the counterpart of unusable one which is
System fuel is the weight of all fuel required to fill all lines and tanks up to the drain point of the tank. The unusable fuel is the amount of fuel in the tanks which is unavailable to the engines under critical flight conditions as defined in FAR Part 25.959.
AS FAR AS FAR CONCERNS. EU OPS will have it somewhere as well. Legally single engine you're entitled to call out MAYDAY any time and deviate from any given rules and procedures in the interest of safety.
