An unattached athlete is one that has no affiliation with a Team. An unattached athlete's points will not be accumulated towards Team Score, but will count for Individual High Point scores.
When you Add or Edit an Athlete, MM allows you to assign that athlete to an "unattached" team. An unattached Team is any Team that has the first 3 characters of the Team Abbreviation as "UN-". For example, "UN-FL" and "UN-CA" would both be unattached Team names. In addition, "UNAT" and UNATT" are also reserved as Unattached Team Abbreviations. Any athletes on teams using any one of these Team Abbreviations would be classified as "unattached".
USA Swimming uses "UN" as their official Unattached designation for Unattached athletes. When using the Exceptions Report's Registration ID check, it will match the "UN" with any athlete using one of MM's official unattached team abbreviation codes.