Hynek worked out a classification system, where he made a distinction between distant encounters and close encounters.
- Distant Encounters:
- Nocturnal Lights (NL): Anomalous lights seen in the night sky.
- Daylight Discs (DD): Any anomalous object, generally but not necessarily "discoidal", seen in the distant daytime sky.
- Radar/Visual cases (RV): Objects seen simultaneously by eye and on radar.
- Close Encounters (I, II, and III):
- CE-I (Close Encounters of the First Kind): Strange objects seen nearby but without physical interaction with the environment.
- CE-II (Close Encounters of the Second Kind): A CE-I case that leaves physical evidence, e.g. soil depressions, vegetation damage, or causes electromagnetic interference.
- CE-III (Close Encounters of the Third Kind): CE-I or CE-II cases where occupants or entities are seen.
Hynek did NOT consider contact experiences cases of CE-III:
"In these cases [CE-III] the presence of 'occupants' in or about the UFO is reported. Here a sharp distinction must be made between cases involving reports of the presence of presumably intelligent beings in the 'spacecraft' and the so-called contactee cases. In general, the latter reports are "stopped at the gate" by the screening process…. Such persons not only frequently turn out to be pseudo religious fanatics but also invariably have a low credibility value, bringing us regular messages from the "space men" with singularly little content" [Hynek, The UFO Experience, p. 29.]
Expansions of Hynek's classification
The term CE-IV, or close encounter of the fourth kind, is often used to refer to abduction experiences. Author C.D.B. Bryan, who wrote a book with the title 'Close Encounters of the Fourth Kind', in which he defines a CE-IV as:
"... personal contact between an individual or individuals is initiated by the 'occupants' of the spacecraft. Such contact may involve the transportation of the individual from his or her terrestrial surroundings into the spacecraft, where the individual is communicated with and/or subjected to an examination before being returned. Such a close encounter is usually of a one-to-two-hour duration." (Bryan, p. 9.)
Note that Bryan does not limit CE-IV to abductions only. Contactees qualify too as CE-IV cases as they are cases of personal contact with the occupants of a spacecraft and initiated by those occupants.
Later on, a fifth type was added, CE-V or close encounter of the fifth kind, which is described as a conscious human-initiated contact with extraterrestrial intelligence. The difference between CE-IV and CE-V lies in who takes the initiative for the encounter: in CE-IV cases, the initiative is taken by the occupants of the spacecraft, whereas in CE=V cases, the initiative is taken by the witnesses, and happens at their request.
Jacques Vallee created his own classification that is also fairly commonly used. The focus is on UFOs only, and uses four main categories with 5 subcategories each.
The four main categories are:
- Close Encounter (CE): As per Hynek.
- Maneuver (MA): Trajectory discontinuity inflight.
- Fly-by (FB): No observed discontinuity in flight.
- Anomaly (AN): Unusual lights or unexplained entities.
The five subcategories are:
- Physical effects: for example, radar sighting
- Life form or living entity
- Reality transformation: witnesses experienced a transformation of their sense of reality (often corresponding to the popular characterization of the incident as an abduction)
- Physiological impact: Such as death or serious injury
These five subcategories can apply to all four of the main categories of sightings.
Thus, the Vallée categorization categorizes cases as MA-2, AN-1, CE-4, for example.