About single-criteria relationships
Number field, auto-enter serial number; this will be the match field in the Customers table In the Edit Relationship dialog box, the Allow creation of records in this table via this relationship option is enabled. This means that you will be able to enter a phone number in an empty portal row, and FileMaker Pro will automatically create a related record for that phone number. Also, the Delete related records in this table when a record is deleted in the other table option is enabled, because there is no reason to retain old phone numbers if the matching contact record has been deleted.