The identifier collected by the FRB is an alpha-numeric code of up to 32 characters. The FRB provides the following instructions in the FR Y-14M for the loan number field:

An identifier for a loan that will be the same from month to month. Reference numbers may be used in lieu of actual loan numbers as long as it meets these criteria. This loan identifier must uniquely identify any loan in the file. It must identify the loan for its entire life and must be unique (piggy-backs should be separated).

If the bank holding company (BHC) is already submitting data to the OCC as part of the OCC Mortgage Metrics Data or OCC Home Equity Data, it is required that the BHC use the same loan number for the FR Y-14M data schedules. The FR Y-14M data population may include additional loans, which may not be part of the OCC data sample, and for such loans the general requirements listed in the schedule instructions will be applicable. Overall, the entire data file sent by a BHC should have unique loan numbers across the entire submission.