To ensure that only the sales reps and their manager's executive can view the rep's evaluation record but the reps should not be able to view the executive comment field on their review, a private sharing model granting record access using hierarchy and field-level security should be used. A private sharing model means that only the owner of a record and users above them in the role hierarchy can view the record by default. Record access can be granted to other users using manual sharing, sharing rules, or Apex sharing. Field-level security controls which fields are visible or editable for different profiles or permission sets.