The apparent field of view is a value in degrees that represents the viewing angle of an image you see through the eyepiece. Two binoculars that share the same magnification, lens diameter, and field of view don’t necessarily have the same apparent field of view, because it depends on the structure of the lenses inside an eyepiece. It’s simply a subjective impression of the field of view.
The apparent field of view is also depending on the eye relief distance. Shorter eye relief means a wider apparent field of view. But if comparing two different binoculars with the same eye relief, the one with the larger eye lens in the eyepiece will have a larger viewer field.
It can be calculated by multiplying the actual field of view with the scope’s magnification. A higher value is better as it makes the image appear wider and bigger.