Datum is simply a reference point designated by the manufacturer to base all the arm lengths off of, and subsequently to determine the "inches aft of datum" in the computation of the aircraft's CG. I'm sure there's some logic to where the manufacturers pick their datum point; such as the nose of the aircraft so all numbers are positive or a prominent structural feature (e.g., the firewall in a Cessna 172). Regardless, it's simply a reference point.
Originally Posted by sellener
Ok just had a thought. Would the Datum be the CG for the empty aircraft prior to loading?? That would make sense to me right now.
No, because each aircraft technically has it's own unique BEW and Moment (ergo it's own unique CG), the datum would theoretically be different for every plane. If that were the case, the arm lengths would be different, so the CG computation would need to be tailored to each aircraft's POH. Not really a viable method of selecting a datum point.