Identical Piece —An individual mailpiece that has the same mail classification and physical aspect, size, and weight as all other pieces in a mailing.

 

Indicia —Imprinted designation on mail that denotes postage payment (e.g., metered postage or permit imprint).

 

Information-Based Indicia (IBI) —Digital indicia that include human-readable information and a USPS-approved two-dimensional barcode with a digital signature and other required fields.

 

Insert —A letter, card, or similar item placed inside another mailpiece (host piece).

 

Insured Mail —A service that provides indemnity coverage for a lost, rifled, or damaged article, subject to the standards for the service and payment of the applicable fee. Insurance is available for merchandise sent as First-Class Mail, Priority Mail Express, Priority Mail, Standard Mail, and Package Services.

 

International Mail Manual (IMM) —The USPS manual that contains prices and classification standards for mailing between the United States and all other countries. The IMM Index of Countries contains complete price information for each country.

 

International Standard Book Number (ISBN) —A publication number issued by the Library of Congress that identifies a specific book or other nonperiodical.

 

International Standard Serial Number (ISSN) —A publication number issued by the Library of Congress that identifies a specific book or other periodical.