UFO CLASSIFICATION SYSTEMS
Hynek Classification System 
Possibly the most famous UFO classification system, this is a system which does not classify reports based on the nature or behavior of the observed object, but instead simply on the visibility or proximity of the object. As such, it is significantly less useful than either of the Vallee classification systems.
Class Name Description
• NL (Nocturnal Light) Point or extended luminous source observed at night.
•DD (Daylight Disc) Metallic or whitish object seen in the day.
•RV (Radar / Visual) Observation supplemented with radar.
•CE-I (Close Encounter I) Observation of an object in close proximity to the witness (i.e. within 500’)
•CE-II (Close Encounter II) Observation of an object in close proximity to the witness, where physical traces (impression, burn, medical effect, etc.) are left or (electrical effect, heat) are felt
•CE-III (Close Encounter III) Close observation with animate beings associated with the object.
•CE-IV (Close Encounter IV)  Abduction of the witness or other direct contact
Vallee’s Original Classification System 
This is the final form of Jacques Vallee’s original classification system. It still remains one of the best and most descriptive classification systems, particularly when the subtypes are used.
Class Name Description
Type - I (a, b, c, d) Observation of an unusual object, spherical discoidal, or more complex in form, on or close to the ground (no higher than tree height), which may be associated with traces - physical effects of a thermal, luminous, or mechanical order.
a - On or near ground
b - Near or over body of water
c - Occupants displaying interest in witnesses by gestures or luminous signals
d - Scouting a terrestrial vehicle
Type - II (a, b, c) Observation of an unusual object with vertical cylindrical formation in the sky, associated with a diffuse cloud. This phenomenon has been given various names such as cloud-cigar or cloud-sphere.
a - Moving erratically through the sky
b - Object is stationary and gives rise to secondary objects
c - Object surrounded by secondary objects
Type - III (a, b, c, d, e) Observation of an unusual object of spherical, discoidal or elliptical shape stationary in the sky.
a - Hovering between two periods of motion with "dead-leaf" descent, up and down, or pendular motion
b - Interruption of continuous flight to hover and then continue motion
c - Alters appearance while hovering - i.e. change of luminosity, generation of secondary object, etc.
d - "Dog-fights" or swarming among several objects.
e - Trajectory altered during continuous flight to fly slowly above a certain area, circle, or suddenly change course
Type - IV (a, b, c, d) Observation of an unusual object in continuous flight.
a - Continuous flight
b - Trajectory affected by nearby conventional aircraft
c - Formation flight
d - Wave or zig-zag trajectory
Type - V (a, b, c) Observation of an unusual object of less definite appearance, appearing not fully material or solid in structure
a - Extended apparent diameter, non-point source luminous objects
b - Star like objects, motionless for extended periods
c - Star like objects rapidly crossing the sky, possible with peculiar trajectories
Current Vallee Classification System 
This is Vallee’s attempt to unify his classification system with Hynek’s, and to incorporate those "psychic" or otherwise anomalous reports which he believes have a connection with the UFO phenomenon, and to regularize the classification system.
1. Jacques and Janine Vallee: Challenge To Science: The UFO Enigma, LC# 66-25843
2. Dr. J. Allen Hynek, The UFO Experience, ISBN 0-345-27361-3,
3. This category was not explicitly approved by Dr. Hynek, but is in wide use.
4. Jacques Vallee, Confrontations, ISBN 0-345-36501-1