Deciphering USCIS Case Receipt Number
Whenever you a file a petition (such as I-485, Adjustment of Status) with USCIS, you get a Receipt Notice (Notice of Action - Form I797-C) that has a 'Receipt Number'. The characters in the receipt number has significance. Actual meaning of such characters depends upon the software system by USCIS to process the case.

CLAIMS3 - LAN Version
Example of such receipt number is
EAC 08 015 50023

First three letters represent the service center received the case.

There are four service centers:
EAC - Vermont Service Center (Previous name: Eastern Adjudication Center).
LIN - Nebraska Service Center (LIN refers to Lincoln, Nebraska).
SRC - Texas Service Center (Previous name: Southern Regional Center).
WAC - California Service Center (Previous name: Western Adjudication Center).

08 - Represents the fiscal year in which the case was received. Fiscal year 2008 runs from Oct 1, 2007 to Sep 30, 2008.

015 - Represents the computer workday in the fiscal year on which the fee was taken. 001 is October 1, the first working day of the fiscal year. 015 represents 15th working day which was actually Oct 19, 2007. Computer work day is really a business day and it excludes most Saturdays, all Sundays and holidays. Towards the end of the fiscal year in September, the number goes up to around 265.

50023 - Represents the particular case number. The number always starts with 5 and it has a total of five digits.

Therefore, this receipt number means: this case was the 23rd case received in CLAIMS3 LAN on the 15th computer work day of fiscal year 2008 at Vermont Service Center.

Most cases filed at USCIS have this type of receipt number, except few as described below.

You can track the status of this type of case online.

CLAIMS3 - Mainframe
Example of such case number is:
EAC 08 015 00023

EAC - Look above in CLAIMS3 - LAN Version.

08 - Look above in CLAIMS3 - LAN Version.

015 - Look above in CLAIMS3 - LAN Version.

00023 - Represents the particular case number. In contrast to the cases received in CLAIMS3 - LAN version, the cases received in Mainframe version start with 0.

You can't track the status of such a case online.

N-400 (Naturalization application) is processed in this system.

The structure of this case numbers is totally different than the ones described above. It has two significant numbers.

Application identification number. It is a series of letters, numbers, and a symbol.

e.g., EAC*000040974

These numbers are strictly sequential and they don't get rest at the end of the fiscal year.

You need to refer to this number when seeking information regarding the status of the case.

Payment Identification Number. This number is printed on the reverse of the financial document (such as check or money order) that was used to pay the fee.

e.g., EAC$000065098.

These numbers are strictly sequential and they are not the same number as the 'APP ID'.

This number may be followed by some other numbers: e.g.,
EAC$000065098 (1)
EAC$000065098 (2)

In case if you used more than one financial document to pay for the application, you may see one number for each payment. e.g., you may have sent two checks for N-400: one for application fee and another for fingerprinting fee.

Anyway, the case number is still only the APP ID. That is, EAC$000065098 in this case.

You can't use PAY ID for finding the case status in the automated information system. You must use the APP ID for that. However, while talking with a USCIS representative on the phone, you may use "PAY ID" as well to seek case status information.