Usually - for Divisions and Regiments - these numbers consist of 5 numbers in a XabXc-pattern. The first and fourth number = XX minus 11 denotes the Division a type is assigned to. For example: the J-11BS numbered "10326" has XX = 12, so 12 - 11 = 1. Fighter Division.
The numbers abc - aka the second, third and fifth number - are for 3 blocks of 50 each and are denoting the Regiment: abc = 001 to 049 = first Regiment within a Division, abc = 051 to 099 = second Regiment within a Division and finally abc = 101 to 150 = third Regiment within a Division.
The newer system for the Bases and their subordinated Brigades is a bit different. Here the pattern is of a XXaXb system, where XXX minus 611 denotes the Brigade and the numbers 'ab' represent the individual aircraft-number within that Brigade.
For example: the J-20A numbered "78275" has XXX = 787, so 787 - 611 = 176. Brigade and the "ab" = 25 means aircraft no. 25 within Brigade, however not the 25th J-20A.
Hope that helps.
Deino
By the way, I'm away for one week from tomorrow morning; a school-trip with my class into the deepest forest without any internet-access ... I'm sure during these days, FC-31V2 will fly again, the J-15A or B will be unveiled and a lot more ... as usual when I'm off-line.
All the best.