Hynek system and Vallee system are two most common method of categorization of UFO. J. Allen Hynek developed his original system in the 1970s. It is used to describe sightings of UFOs and classifies the sightings into 6 groups. The system groups sightings into two major types first: distant and close encounters. The system has a cutoff point as big as five hundred feet.
The close and distant encounters are then divided into further categories using special features and appearance as the basis. Special features in question include Nocturnal Lights (NL; refers to anomalous lights observed in the night sky), Daylight Discs (DD; any object of anomalous nature generally “discoidal” seen in the distant daytime sky), and Radar/Visual cases (RV; objects are observed and confirmed simultaneously by eye and on radar).
The close encounter (CE) subcategory is further divided into three subgroups. CE1 is when strange objects are seen nearby but they do not make physical interaction with the environment. CE2 is when physical evidence exists such as electromagnetic interference, radiations, vegetation damage, or soil depressions. CE3 is when CE1 or CE2 occurs along with visible occupants or entities. CE4 was introduced later by Hynek, describing cases that involve witness feeling abducted by a UFO. CE5 was developed by some ufologists to describe human-initiated contact with ET intelligence.
Vallee categorized UFO sightings into 4 types: Close Encounter (CE; similar to Hynek), Maneuver (MA; flight’s trajectory discontinuity), Fly-by (FB; absence of discontinuity in flight), and Anomaly (AN; atypical lights or unexplained entities). Vallee adds five more subcategories that apply to the previous four: sighting, physical interference (e.g. radar sighting), life form or living entity, transformation of reality (witnesses’ sense of reality is transformed, often characterized as abduction incident), as well as impact of physiological nature such as serious injury or death.