With reference to mass and balance calculations (on an aeroplane) a datum point is used. This datum point is:
a point near the centre of the aeroplane. It moves longitudinally as masses are added forward and aft of its location
the point through which the sum of the mass values (of the aeroplane and its contents) is assumed to act vertically
a fixed point from which all balance arms are measured. It may be located anywhere on the aeroplane's longitudinal axis or on the extensions to that axis
a point from which all balance arms are measured. The location of this point varies with the distribution of loads on the aeroplane