From the user guide:
Fit ellipse Fits an ellipse to the selection. Uses the headings Major, Minor and Angle. Major and Minor are the primary and secondary axis of the best fitting ellipse. Angle is the angle between the primary axis and a line parallel to the X-axis of the image.
Feret’s diameter The longest distance between any two points along the selection boundary, also known as maximum caliper. Uses the heading Feret. The angle (0–180 degrees) of the Feret’s diameter is displayed as FeretAngle, as well as the minimum caliper diameter (MinFeret).
Does that help?