last 3 numbers in Administrative Processg. Case #

[benz]

Hello, Does anyone know what does the last three numbers of the case # mean, first 7 numbers are clear - yer followed by the day of the year interview took place.
What is the 3 number code after that example 997-1 or 998-1 etc etc.
Does anyone know how to identify the student visas vs visitors vs H1 renewals etc etc