Certification accompanies mail to the Post Office. You get a copy. Certification verifies that addresses match the Post Office national database of deliverable mail.
Unmatched addresses mail first class, or optionally, eliminated from the mailing.
Certification does not insure accuracy, only deliverability. A record with no name is a duplicate if the address matches. A record with no name is a duplicate if the address matches. For one address per company. Residentials are not affected.