A Standard Carrier Alpha Code (SCAC) is a unique two-to-four-letter code used to identify transportation companies. The SCAC code was developed in the 1960s by the National Motor Freight Traffic Association to help road transport companies computerize data and records. SCACs are commonly used by various industries, including the automobile, petroleum, forest products, and chemical industries, as well as suppliers to retail businesses, carriers engaged in railroad piggyback trailers, and ocean container drayage. Freight carriers who participate in the Uniform Intermodal Interchange Agreement (UIIA) are required to maintain a SCAC. Certain groups of SCACs are reserved for specific purposes, such as codes ending with the letter "U" for the identification of freight containers and codes ending with the letter "X" for the identification of privately owned railroad cars. SCACs are also used to identify an ocean carrier or self-filing party, such as a freight forwarder, for the Automated Manifest System used by US Customs and Border Protection for electronic import customs clearance and for manifest transmission as per the USAs "24 Hours Rule" which requires the carrier to transmit a cargo manifest to US Customs at least 24 hours prior to a vessels departure at port of loading.