Datum reference

A datum reference or just datum (plural: datums[Note 1]) is some geometrically important part of an object—such as a point, line, plane, hole, set of holes, or pair of surfaces—that serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value.

This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly).

In aviation, an aircraft is designed to operate within a specified range of mass and (chiefly longitudinal) balance; an airman is responsible for determining these factors for each flight under their command.

This requires the calculation of moment for each variable mass in the aircraft (fuel, passengers, cargo, etc.

The datum for light airplanes is usually the engine firewall or the tip of the spinner, but in all cases it is a fixed plane perpendicular to the aircraft's longitudinal axis, and specified in its operating handbook.

An engineering datum used in geometric dimensioning and tolerancing is a feature on an object used to create a reference system for measurement.

[1] In engineering and drafting, a datum is a reference point, surface, or axis on an object against which measurements are made.

Finally, The edge datum C controls 1 degree of freedom, namely the translation along the x-axis.