https://about.usps.com/publications/pub8/pub8_v05_revision_092017_006.htm
ACS is an enhancement to the existing manual address correction process provided with the use of Ancillary Service endorsements on all mail classes except Periodicals. The following ancillary services are available for use with ACS on First-Class Mail, USPS Marketing Mail, Package Services, and Parcel Select:
- Address Service Requested provides forwarding and address correction services when possible. If the mail is undeliverable and not forwardable, it is returned to the sender with the reason identified on the mailpiece.
- Change Service Requested provides address correction services without forwarding or return. If undeliverable, either the new address or the reason it is undeliverable is provided to the sender.
- Return Service Requested provides address correction services and always returns the piece.