lin number denotes
the financial year and the serial number of case received in that year.
Hence you know that if INS has processed say 1000 and yo are 1051, decision on your case is not far away.
------ from immigration-law.com
Illustration of System (1) Notice: LIN-99-001-50001: Most of visitors to this web site receive this system of notices.
VSC, TSC, NSC, CSC: People should know what these stand for: Vermont Service Center, Texas Service Center, Nebraska Service Center, California Service Center
99: Represents the fiscal year the case is receipted.
001: The first day of the fiscal year, 10/01/FY. It only counts so-called computer working days excluding week-ends and holidays. Generally the ending date of any FY is 265, 09/FY.
50001: The number "5" stands for symbol of this receipt notice system as opposed to other two systems. The whole number represents the particular case number. In this case, it is the first case receipted into the system on that particular workday.