This is left-over from when the ATF used to be part of the Department of Treasury.
The second section of an FFL number is a two-digit number which represents the IRS district where the FFL is located. The regions and their corresponding numbers (which skip numbers 2 and 7) are: The first section of an FFL number is a single digit which represents which of the 7 regions of the country where the FFL is located.
