This drop-down column becomes available when the selected Met By value is defined as Reference Required, which is almost always a Substitution (S). The list contains only those Advising Requirement Codes (ARCs) with a Cluster Code of C (Course) or CC (Cumulative Credit). In addition, the drop-down list will only display ARCs for courses which the student has already taken.
The default behavior of this column filters the list of ARCs to display only those that are specific to the selected student; however, if you want to project substitutions into the future (for courses that the student has not yet taken), select the Show All Course Requirements checkbox.