19 C.F.R. § 145.73
For purposes of this subpart:
Designated operator means an entity officially designated by a member country of the UPU to operate postal services and fulfill its treaty obligations to the UPU. USPS is thus considered a designated operator for the United States.
Express Mail Service or EMS means the optional supplementary postal express service for documents and merchandise.
International Mail Facility or IMF means an official international mail processing center operated by CBP.
Item ID means the unique item identifier, in both human-readable and barcode format.
Letter class mail-documents means letter class (in UPU terms, letter post) mail containing only documents. Documents consist of any piece of written, drawn, printed or digital information, excluding objects of merchandise and may include M-Bags to the extent that such items do not contain goods.
Letter class mail-goods means letter class (in UPU terms, letter post) mail up to 2 kilograms containing goods, also referred to as "small packets". Mail over 2 kilograms containing goods must use a postal service other than letter class.
Parcel post means any mail article mailed at the parcel rate or equivalent class or category of postage.
Universal Postal Union or UPU means the specialized agency of the United Nations that sets the rules for international postal service for member countries.
19 C.F.R. §145.73