The navicular drop test is a clinical test that measures how much the navicular bone in the foot drops between neutral position and relaxed or between relaxed and full weightbearing.
The navicular drop test only measures sagittal plane excursion of the navicular bone. It does not measure transverse plane motion of the navicular, so it is limited by that.
The navicular drop test is assumed to be a measure of the “pronation” of the foot by quantifying the “collapse of the arch”. As foot pronation is a combination of many factors, just relying on the navicular drop test to quantify it is going to be limited.