When you draw an ellipse, its shape is determined by two axes that define its length and width: the major (longer) axis and the minor (shorter) axis.
The illustrations below show two different ellipses created by specifying axis and distance. The third point specifies only a distance and does not necessarily designate the axis endpoint.
If you are drawing on isometric planes to simulate 3D, you can use ellipses to represent isometric circles viewed from an oblique angle.