This is a field marked by the star * symbol, which has to be filled in order to make the record completed.

A mandatory field is necessary to ensure the proper routing and or posting on an ACH entry. Any mandatory field not included in an ACH record will cause that entry batch or file to be rejected by the ACH operator before it ever gets sent to the RDFI.

a field within the form that must be completed to allow the Scheme to deal effectively with the information you are providing