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Under the NACHA Operating Rules, checks with an Auxiliary On-Us field in the MICR line are ineligible for ARC, POP, or BOC conversion. Such checks can often be identified because they are larger - up to 8-3/4 inches long - than six-inch checks. However, not all large checks are ineligible - only those with an Auxiliary On-Us field qualify. What is the Auxiliary On-Us Field?The Auxiliary On-Us field appears in the leftmost position of the MICR line of large-sized checks. If the Auxiliary On-Us field is used, it must be positioned to the left of the Routing Number or, if an External Processing Code (EPC) is included in the MICR line, the Auxiliary On-Us field must be placed to the left of the EPC.  For more specifics on how the Auxiliary On-Us field is populated and used, please download the PDF: Auxiliary On-Us Field: What It Is, Why It's Important. This document contains detailed examples of the MICR encoding on checks with and without the Auxiliary On-Us field. |
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