USPS Intelligent Mail (aka: OneCode) is a height-modulated 4-state barcode type used in the USA. It allows both PLANET and POSTNET barcode information to be combined into a single barcode to track mailings, request address-quality services (including updated address-change information) and return-mail service.
Intelligent Mail Barcode Index:
Intelligent Mail and OneCode are the USPS implementations of the 4-State Customer Barcode type. Other implementations of the 4-State Customer Barcode exist in other countries such as RM4SCC, which is used for automated mail sorting and addressing in the UK.