Ancillary Endorsements provide the USPS instructions on how to handle forwarded and undeliverable-as-addressed pieces. They are the same for all classes of mail, but the treatment and cost differ by mail class. Use of an ancillary endorsement on a mailpiece obligates you to pay USPS any applicable charges for forwarding, return, and separate address notification charges. Details can be found here: USPS
Important: per USPS Move Update requirements, your use of ancillary endorsements obligates you to update address records with all changes that you receive back from USPS.