Maybe it is a body style/ model designation.

For example:
pre 1985 Jeep Wranglers were know as CJ
1985 - 1995 Jeep Wranglers are known as YJ
1997 - pres Jeep Wranglers are known as TJ
Old postal jeeps where DJ

There where also additional numbers or letters to designate small changes. CJ2A, CJ2B, CJ3A, etc But basicly for Jeeps it designates the changes in the body designes.