Use the Select Datum window to select the datum and 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.
For the United States the most commonly selected datum is WGS-84 1984 (NAD-83).
! If your datum is not listed here - such as NAD-27 or some outside the U.S. - use the new Custom tab to select yours or define a custom projection.
See also