http://www.usps.com/ncsc/addressservices/moveupdate/ace.htm

Ancillary service endorsements allow the sender to obtain on request (provided the appropriate endorsement is used) the addressee's new (forwarding) address (if the addressee filed a Change-of-Address Order with the Postal Service) or the reason for non-delivery. These endorsements also provide the Postal Service with instructions for the disposition of UAA mail. These new endorsements provide a simpler and more consistant system than the previous endorsements.

Undeliverable-as-addressed [UAA] mail is forwarded, returned to sender, or treated as dead mail as authorized for the particular mail class. An mailer endorsement is used to instruct the Postal Service® regarding the mail piece's appropriate disposition upon determining that it is UAA. On First-Class Mail®, the following endorsements may be used as an updating method to meet the move update standard:

• Return Service Requested. If UAA, the mail piece is returned with the new address or reason for non-delivery; no charge.
• Temporary Return Service Requested. If UAA and a temporary change-of-address has been filed, the mail piece is forwarded at no charge. No separate notice of anew temporary change-of-address is provided.
• Address Service Requested.
• Months 1 - 12: the mail piece is forwarded; no charge; a separate notice of the new address is provided; an address correction fee is charged.
• Months 13 - 18: the mail piece is returned with the new address attached; no charge.
• After month 18 or if undeliverable: the mail piece is returned with reason for non-delivery attached; no charge.
• Change Service Requested. Separate notice of new address or reason for non-delivery provided; in either case, address-correction fee is charged; mail piece is not forwarded or returned but disposed of by the Postal Service. This endorsement option is available for First-Class Mail only when used in conjunction with electronic Address Change Service [ACS].
• Optional Ancillary Service Endorsement Locations. In addition to the current location under the return address (1), ancillary service endorsements may be placed above the delivery address block (2), to the left of the postage area (3), or below the postage area (4) (postage stamps, meter stamps, or permit imprints). See DMM M012.4.3 for more information.