A bit of background from Bishop's piece:
His 1972 book The UFO Experience: A Scientific Inquiry contained his proposal for a system of classifying UFO sightings. The system was envisioned as a first step in legitimizing UFO research as a valid scientific pursuit. When a witness is less than 500 feet from the observed phenomena, this was labeled a Close Encounter. The distance from the observer was important because, depending on the apparent size of the observed anomaly, details may be less obvious and mistaken for something truly unknown.
You can read the entire article here.