This field is on the Direct Mail Relay Item Report (RDM). If you selected Y in the Update User Codes field, then you must select which user code(s) you want to update in the User Codes section (in the bottom half of the window).
In the boxes on the left, type the number(s) or letter(s) that you would like the system to use as the user code(s). In the boxes on the right, type an "X" in the box(es) that corresponds to the boxes you filled in on the left.