Use the Select Datum window to select the datum or reference ellipsoid upon which your UTM coordinates are based. Listed here will be many of the "standard" datums worldwide upon which hundreds of minor datums were derived. The parameters of the ellipsoids (equatorial radius, polar radius, and inverse flattening) are displayed within the window.
An ellipsoid is a mathematically defined surface which approximates the geoid: the surface of the Earth's gravity field, which is approximately the same as mean sea level.
For the United States the most commonly selected datum is WGS-84 1984 (NAD-83).
See also