:'''u''' - '''Unknown'''. Used when codes are being assigned, but the appropriate specific value cannot be determined.

:'''x''' - '''Not applica'ble'''. Used when a characteristic is not appropriate for the type of entity being described.

:'''z''' - '''Other'''. Used when codes are being assigned and the characteristics of the entity are known, but none of the defined codes is appropriate.

:'''|''' - '''Fill character'''. Used when no attempt is being made to assign the codes.