We often use a Gift Sub-Type to designate alternate GL posting for various cash gifts or pledges.  For instance, we have a gift sub-type for funds given through payroll contribution that debits a "payroll clearing" account instead of our bank account.

There are times when we would like to track the reason for a write-off through GL accounts.  For instance, a write off because a donor indicated the pledge would not be honored may be posted to one account versus a write off because we have lost contact with a donor.  

The GL Distribution tab in a Fund record allows Sub-Types to be added under the Pledge-Write Off  Gift Type, but I don't see that those Sub-Types can be used when writing off a gift, including when importing write-offs.