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For balance purposes and Centre of Gravity(CG) calculations, the imaginary vertical plane from which all horizontal distances are measured.
The reference datum for an given aircraft type is defined by the manufacturer. It may be at a specific aircraft station or, in many cases, it may be at a specified distance forward of the aircraft nose to simplify centre of gravity calculations. Arms and moments are calculated based on the reference datum.
ICAO Annex 8: Airworthiness of Aircraft.
US FAA Aircraft Weight and Balance