CSI, an initiative for improved container security, is also associated with the 24 Hour Rule. CSI is an agreement between U.S. Customs and governments, customs authorities and ports aiming to ensure early identification of containers that pose a high risk. CSI requires the use of high-security seals and transmission of cargo documents in electronic form to U.S. Customs 24 hours before loading.
U.S. Customs FAQ: Additional Frequently Asked Questions (link)