What does first letter in file number stands for .. PMM pls

[akrk]

I have seen most of ppl file number start with F under family category.
Mine, however starts with B.

Wondering if there is any logic in the first letter of the file number ?

 

[PMM]

Hi

I have seen most of ppl file number start with F under family category.
Mine, however starts with B.

Wondering if there is any logic in the first letter of the file number ?

No, they started with "A" and for a few years has been "B" when that runs out, it will be on to 'C"