External Data always shows preview of first 10 rows of data.
If you specify the External data as 2D, when importing into a Mechanical model set as 3D, you can get rid of one of the XYZ (or R,theta,Z) coordinates, and have it sweep values in the missing direction. The sweep direction is chosen in Mechanical for the "2D Projection" field. It is possible to sweep in a linear or angular direction, but not the radial direction of a cylindrical coordinate system. For example, you can get rid of the Z coordinate by setting the External Data system to 2D and setting the sweep direction to Z in Mechanical using "Normal to Plane." You can get rid of another specification of coordinate by entering a fixed value in the "Analytical Transformation" section. If you don't specify a fixed value in the "Analytical Transformation" section, but still leave out that column of coordinate location in the data, then it will assume a value of zero if not the 2D sweep direction. Turning on display of source points in Mechanical will show it chose a Z coordinate of zero when none was given.
In simple models, a large pinball radius on the Imported Load can accomplish a similar thing as sweeping 2D data to 3D model.