Strictly speaking, an ellipse is a figure that has 2 centers called foci. The sum of the distance from any point of the ellipse to one of the foci, plus the distance from that same point to the other focus, will always be equal to the same sum of any other point of the ellipse. This is its classical definition. However, to build an ellipse with Autocad, it is not necessary to determine the foci. The geometry of the ellipse can also be composed of a minor axis and a major axis. The intersection of the major axis and the minor axis will be, at least for Autocad, the center of the ellipse, so a method to draw ellipses with full precision is by indicating the center, then the distance towards the end of one of the axes and then the distance from the center to the end of the other axis. A variant of this method is to draw the start and end point of one axis and then the distance to the other.
On the other hand, the elliptical arcs are ellipse segments that can be constructed in the same way as an ellipse, only that at the end we must indicate the initial and final value of the angle of said arcs. Remember that with the default configuration of Autocad, the 0 value for the angle of the ellipse coincides with the major axis and increases anti-clockwise, as can be seen below: