The HCU-6/E or 463L Master Pallet is a standardized pallet

Pallet

A pallet , sometimes called a skid, is a flat transport structure that supports goods in a stable fashion while being lifted by a forklift, pallet jack, front loader or other jacking device. A pallet is the structural foundation of a unit load which allows handling and storage efficiencies...

 used for transporting military air cargo. It is the main air-cargo pallet of the United States Air Force

United States Air Force

The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

, designed to be loaded and offloaded on today's military airlifters as well as many civilian Civil Reserve Air Fleet

Civil Reserve Air Fleet

The Civil Reserve Air Fleet is part of the United States's mobility resources. Selected aircraft from U.S. airlines, contractually committed to Civil Reserve Air Fleet, support United States Department of Defense airlift requirements in emergencies when the need for airlift exceeds the capability...

 (CRAF) cargo aircraft.
The "SS-463L" project was developed by a U.S. Air Force committee in 1957 and awarded to the Douglas Aircraft Company in 1959.