A Facing Identification Mark - FIM - is a small group of vertical bars that the USPS prints near the top edge of reply mail, left of the postage area. Sorting machines use it to turn each letter the right way up and to tell what kind of reply mail it is. There are four of them, FIM A to FIM D; the USPS says which one a mail piece needs.

FIM A	FIM B
FIM C	FIM D

What it holds
Characters	One letter, A, B, C or D, in capitals or not. FIM E is accepted too.
Check digit	None.
Readable text	None: Text position isn't offered.

The rest of the settings - quiet zone, color - are in the Barcode section.

What goes wrong
Invalid characters in input data.	The letter isn't A to E.
Invalid number of characters in the input data.	More than one letter.