When investigating UFO and alien sightings in the past it became apparent that there was a need for a structured system for everyone to use. There are currently two different systems that are widely used by ufologists the Hynek System, and the Valle System named after the people that came up with them.
The Hynek System:
The Hynek system is the less used system today but probably the most known by non ufologists.
Dr. Josef Allen Hynek was born in Chicago in 1910 and passed away in 1986. He was an astronomer and astrophysicist that worked with the US Air Force researching UFOs including but not limited to project blue book. He was a brilliant and very well respected scientist that at first didn't believe in UFOs but after years of research with the Air Force he soon changed his mind.
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.
DD (daylight disc): A simple visual sighting of a UFO with distinct shape seen during the day.
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): 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: Broken grass or radiation 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 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.
The Vallee System:
The Vallee system is a more in depth system of classifying UFO and paranormal experiences then the Hynek system and is used more today then the other.
Dr Jacques Vallee was born in 1939 in France and is a well known venture capitalist, computer scientist, ufologist and astronomer. He worked on the staff of the French Space Committee where he saw objects in space that were self propelled and not man made. He also witnessed the destruction of tapes that were recording these objects by higher ups which is what really fueled his love for the search for UFOs.
This is a break down of his UFO and paranormal classification system that he invented to get a better grip on the research of UFOs visiting Earth.
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.
FB RATING Fly-by rating.
FB1 A simple sighting of a UFO traveling in a straight line across the sky.
FB2 FB1 accompanied by physical evidence.
FB3 A fly-by where entities are observed on board (rare).
FB4 A fly-by where the witness experienced a transformation of reality into the object or its occupants.
FB5 A fly-by which the witness would suffer permanent injuries or even death.
CE RATING Close Encounter rating. Similar to the Hynek Close Encounter ratings.
CE1 UFO comes within 500 feet of the witness, but no after effects are suffered by the witness or the surrounding area.
CE2 A CE1 that leaves landing traces or injuries to the witness.
CE3 Entities have been observed on the UFO.
CE4 The witness has been abducted.
CE5 CE4 which results in permanent psychological injuries or death.