Bank identification number often called Bank fraction number. After check 21 law Bank fraction numbers are not much used anymore, it is not mandatory to have it on the check or a check draft to process the check.

The prefix (no longer used in check processing, yet still printed on most checks) is a 1 or 2 digit code (P or PP) indicating the region where the bank is located. … For example, a check from Wachovia Bank in Yardley, PA, has a fraction of 55-2/212 and a routing number of 021200025.

A fractional is used to determine the financial institution of the share draft if the MICR line is ever damaged or torn off:

The fractional number goes like this: XX-YYYY/ZZZZ.

ZZZZ is the first 4 digits of your routing number (you can skip the leading zeroes).

YYYY is the next 4 digits of your routing number (the last, 9th digit, is control digit).

XX is the city/state where the bank is located (I believe it’s based on the original HQ, I know that for WF in California, it’s 11, which is San Francisco). You can find the full list on Wikipedia once the blackout is over (in the meantime – call your Congressman and voice your objections to SOPA).