Although the so called "modern era" of Ufology began with the Kenneth Arnold sighting in 1947, UFO reports go back to ancient times. Even cave drawings, sculptures, paintings, and folklore of yesteryear show an influence of flying machines on mankind.
Because of the many varied reports of UFOs, it seemed necessary to establish a system for classifying these reports. There are two different systems used by Ufologists, the Hynek System, and the Valle System. The former is by far the most widely used;

NL (nocturnal light): a simple visual sighting of a unidentified flying light seen at night.
ND (nocturnal disc): a simple visual sighting of a unidentified flying extended or structured light source seen at night. (This is an extension to Hynek's system).
DD (daylight disc): A simple visual sighting of a UFO with distinct shape seen during the day.
Radar Cases: UFOs detected by Radar.
Radar Visual Cases: UFOs observed visually whilst being simultaneously on radar. From Hynek's study these make up 1 to 2% of reports.
Close Encounters of the first kind (CE1, CEI): as first defined by Hynek, a CE1 is an observation of a UFO within 150 yards.
Close Encounters of the second kind (CE2, CEII): a UFO which leaves some form of physical evidence. Example: A burn where the UFO appeared to touch the ground or the finding of material of unknown makeup.
Close Encounters of the third kind (CE3, CEIII): a visual sighting of an occupant or entity associated with a UFO. An analysis by Hynek of 650 reports found only 1% to be CE3 (Close Encounters beyond the third kind are extensions to the basic Hynek system. Different authors have used the same designation to mean different things.)
Close Encounters of the fourth kind (CE4, CEIV): an abduction of an individual by an alien being or race.
Close Encounters of the fifth kind (CE5, CEV): a direct contact or communication with alien being or race. Example: Billie Meier with the Pleiadians, U.S.Govt. with the Greys, or channeling.
Vallee Classification System:
A system used to categorize the various types of UFO and paranormal experiences invented by Dr Jacques Vallee and is now used more often instead of the Hynek Classification System since it gives the reader a more detailed summary of a case. The categories are as follows...
AN RATING Classifies any anomalous behavior.
AN1 Anomalies which have no lasting physical effects. i.e. amorphous lights, unexplained explosions.
AN2 Anomalies which _do_ have lasting physical effects. i.e. poltergeists, materialized objects, areas of flattened grass, corn circles.
AN3 Anomalies with associated entities. i.e. ghosts, yetis, spirits, elves and other mythical/legendary entities.
AN4 Witness interaction with the AN3 entities. i.e. near-death experiences, religious miracles and visions, OBEs (out-of-body experiences).
AN5 Anomalous reports of injuries and deaths. i.e. SHC (spontaneous human combustion), unexplained wounds as well as permanent healing that results from a paranormal experience.
MA RATING Describes behavior of a UFO. It is analogous to the Nocturnal Light, Daylight Disk and Radar Visual Hynek classifications.
MA1 A UFO has been observed which travels in a discontinuous trajectory. i.e. vertical drops, manoeuvres or loops.
MA2 MA1 plus any physical effects caused by the UFO.
MA3 MA1 plus any entities observed on board. i.e. the airship cases of the late nineteenth century.
MA4 Manoeuvres accompanied by a sense of reality transformation for the observer.
MA5 A manoeuvre that results in a permanent injury or death of the witness.
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