You'll find the info here -
http://www.ins.usdoj.gov/graphics/fieldoffices/scnational/index.htm#F
Your Receipt:
After filing your application and paying your fees, you will receive a receipt number. These receipt numbers are very important. Please use these numbers whenever you write to us about your case, and whenever you contact us to find out about the status of your case.
The numbering system used on receipts has significance.
For most applications filed at a Service Center, your receipt may look like this: LIN 99 001 50001. In this example, the LIN represents the Nebraska Service Center, where the application was initially received into the electronic system. Cases that are filed at or received by another Service Center but processed at the NSC will generally carry the designation of the originating Service Center:
LIN = Lincoln, Nebraska or NSC
EAC = St. Albans, Vermont or VSC
SRC= Dallas, Texas or TSC
WAC= Laguna Niguel, California or CSC
In the example above:
The 99 represents the fiscal year (October 1, through September 30) in which the case was received.
The 001 represents the computer workday in the fiscal year on which the fee was taken.
The 50001 represents the particular case number.
For cases with particular case numbers starting with a 5, electronic case status information is available (see Finding the Status of Your Case, below). For cases with particular case numbers starting with a 0, you must write to request the status of your case.
-Regards
