If an index contains an Unknown value (?), the AVM sorts that value higher than any other value. When you define a unique index, ABL ensures its uniqueness. For example, if cust-num is a unique index, and there is already a cust-num with a value 10, ABL does not allow you to create a cust-num with the value 10. However, ABL does not prohibit users from entering any number of records with unknown values in index fields. You can prevent users from doing this by making the unique index fields mandatory.