The dialog for the creation of the geometry object ellipsoid includes an "axis" field that requires an [x y z] vector which is defaulted to [1 0 0].
The default settings produce the following:
The "axis" vector determines the orientation of the ellipsoid. For these default settings, this vector [1 0 0] is the axis of the largest radii (z, in this case). However, it seems to me that the more intuitive default setting for this "axis" would be [0 0 1] so that the x, y, and z radii settings would be seen/measured along the x-, y-, and z-axis:
The ellipsoid can then be oriented as needed using the "axis" vector:
Maybe I am missing the significance of the "axis" vector... but until I figured it out, I was struggling to design an ellipsoid with the x/y/z/ dimensions that I intended.