Did you know that Japanese license number plates have lots of
information? 

          

  
On the top line, name of the issuing office is indicated mostly by
Kanji and a vehicle class code in numeral.  Tokyo is divided into
five regions, Adachi, Hachioji, Nerima, Shinagawa and Tama.  
You may notice many cars with number plates indicating 品川
in the central and southern area including Izu Islands , 足立 in
the north-east, 練馬in the north-west, and 八王子or多摩 in
the suburb of Tokyo. 

Vehicle Class Codes- 2 digits or 3 digits

         10 – 19, 100 - 198         large trucks

         20 – 29, 200 – 298        buses

         30 – 39, 300 – 396        private cars, over 2000 cc

         40 – 49, 400 – 479        small trucks, less than 2000 cc

         50 – 59, 500 – 579        private cars, less than 2000 cc

         80 – 89, 800 – 879        special purpose vehicles

         90 – 99, 900 – 998        special large sized vehicles

The bottom line contains a Hiragana character and a four-digit
serial number divided into two groups.  There are certain
regulations for using Hiraganas such as れ and わ for rental cars.  
English alphabets are used for vehicles owned by US military
personnel instead of Hiraganas.

Colors are used as another classification;

         White plate with green characters: Private vehicles

         Green plate with white characters: Commercial vehicles

         Yellow plate with black characters: Private vehicles,
                                                                     less than 660 cc

         Black plate with yellow characters: Commercial vehicles,
                                                                     less than 660 cc

These number plates are issued by the Ministry of Land,
Infrastructure and Transport of the National Government for
most vehicles while ones with smaller engines are issued
by the local municipality.

What you should remember in case of emergency are the
4-digit numbers, issuing office and color of the car to help
police identify.

Other types of number plates categorized for official use are:
     Blue plate with white characters: Foreign Diplomats

     White plate with a red diagonal line: Temporary number

     White plate with black characters: Military

Official vehicles of the Imperial household are exempt from
displaying number plates.

For more details, go to an article in Wikipedia:

http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Japan

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