This number is called “aspect ratio” in CCDInspector and here is the definition from the user manual.
Aspect Ratio:
Aspect ratio represents how much out-of-round the star image is. It is the ratio of the longest axis to the shortest axis of the star profile, expressed in percent. A number below 20-30% represents a pretty round star. A number of 0% represents perfectly round star, but you will most likely not see this in real images due to noise and measurement uncertainty.
I discard everything above 14-15%, that’s when eccentricity becomes noticeable.